Magnetic Slime: Making a Fluid Chase a Magnet

Put a strong magnet on the table near a blob of iron-loaded slime and the blob leans toward it, stretches, and then crawls across and swallows it. Children find this unsettling in a way that ordinary slime never manages, and the reason is worth getting right: the slime is not magnetic. Nothing in it has become a magnet. What happens in this magnetic slime experiment is that millions of suspended iron particles are temporarily turned into tiny magnets by the field they’re sitting in, and then pulled toward the place where the field changes fastest. Move the magnet away and the blob goes limp instantly.

Key Takeaways

• You’ll suspend iron oxide powder in a slime base, then use one large magnet to pull, stretch and spike it.
• It teaches magnetic suspensions: a liquid whose stiffness and shape are controlled by a field, which is how real vehicle dampers work.
• Age floor 8, with an adult handling the iron powder and the magnet at all times.
• The usual failure is too little iron. One tablespoon is not enough; two to three is.
• Read the magnet rules in the parent table before you buy anything. Loose small neodymium magnets are a genuine surgical risk for children.

What You’ll Need

Item	How much	Where to get it	If you don’t have it
Black iron oxide powder (magnetite) or fine iron filings	2–3 tbsp, about 40 g	Craft or pottery suppliers sell it as black iron oxide pigment; school science suppliers sell iron filings	Do not use printer toner. It is a fine plastic and carbon powder, a respiratory irritant, and it stains everything permanently
White PVA school glue	120 mL	Stationery aisle	Clear PVA glue works and shows the particles better
Baking soda	1/2 tsp	Baking aisle
Contact lens saline solution	1–2 tsp. Label must list boric acid or sodium borate	Pharmacy	A 2% borax solution, which then needs goggles and the borax precautions
One large neodymium magnet	1, a disc 20 mm across or bigger, or one sealed in plastic	Hardware shop, school supplier	A fridge magnet will not work; it is far too weak. Never use small spheres or a set of small magnets
Disposable gloves	1 pair	Kitchen drawer
Newspaper or a tray	Plenty	Anywhere
Bowl, spoon, measuring spoons	1 each	Kitchen

Nothing here is food and nothing goes near a mouth.

Time: prep 10 min, experiment 30 min, cleanup 15 min Cost: the magnet and the iron powder are the only real costs, and both last indefinitely Mess level: high. Iron oxide stains grout, fabric and fingernails

Do the Experiment: The Magnetic Slime Experiment

Step 1 - Make the slime base, without the iron

Put 120 mL of PVA glue in the bowl and stir in half a teaspoon of baking soda. Add contact lens saline one teaspoon at a time, stirring for 30 seconds between additions, until the mass pulls cleanly off the sides. Knead for two minutes. You want it slightly stiffer than usual, because the iron will loosen it.

What you should see: a normal slime that gathers into one lump. If you want the chemistry of what just happened, our guide to what cross-linking actually means covers the borate bridges in full; here it is just the carrier.

Step 2 - An adult kneads in the iron oxide

Gloves on, newspaper down, and ideally do this outdoors or by an open window. Sprinkle the iron powder onto the flattened slime a tablespoon at a time and fold it in. Do not tip the whole lot in at once and do not let anyone blow on the powder. Keep folding until the colour is uniform charcoal grey with no dry streaks.

What you should see: the slime going grey, then black, and getting softer and heavier as you add more. Two tablespoons gives a clear effect; three gives a strong one. One tablespoon looks black but barely moves, which is the most common disappointment.

Step 3 - Approach with the magnet and find the trigger distance

Put the blob on the tray. Hold the magnet flat on the tray and slide it slowly toward the blob, stopping every centimetre. Note the distance at which the surface first visibly moves.

What you should see: nothing for several centimetres, then a sudden lean and reach at around 1 to 3 cm depending on your magnet and iron loading. The effect is not gradual; it has a threshold, and measuring where that threshold is turns the demo into data.

Step 4 - Let it swallow the magnet, then try to get it back

Put the magnet down about 1 cm from the blob and leave it. Then, when it has been engulfed, try to pull it out with a gloved hand.

What you should see: the blob creeping over the magnet in five to twenty seconds and sealing around it. Retrieving it is genuinely hard, because the slime nearest the magnet has stiffened. Use a plastic fork and expect to lose some slime; this is why the magnet should be one you don’t mind cleaning.

Step 5 - Make spikes, and then prove it is not magnetic

Hold the magnet a few millimetres above the blob. Then take the magnet right away and watch what the blob does over the next ten seconds.

What you should see: sharp conical spikes rising toward the magnet while it is close, then the whole thing sagging back into a puddle within seconds of the magnet leaving. That collapse is the proof: if the slime had become magnetic, the spikes would stay. They don’t.

What’s Actually Going On

The slime is a carrier and the iron is the active ingredient. What you have made is a crude magnetorheological suspension: solid magnetic particles dispersed through a soft medium, where an external field changes the material’s mechanical behaviour. De Vicente, Klingenberg and Hidalgo-Alvarez’s 2011 review in Soft Matter covers the engineered versions, in which applying a field raises the apparent yield stress of the fluid by orders of magnitude within milliseconds.

Here is the mechanism in order. Magnetite particles are magnetically soft, meaning they magnetise readily in an applied field and lose almost all of that magnetisation when the field is removed. So when your magnet comes close, each particle becomes an induced dipole with a north and south end aligned with the field. Two things follow.

First, those dipoles attract each other end to end, so they assemble into chains running along the field lines. Chains spanning the material are what make it stiffer, because deforming the slime now means breaking magnetic chains as well as flowing the gel. That is why the swallowed magnet is hard to pull back out.

Second, and this is the part usually left out, a magnetic dipole in a uniform field feels a torque but no net force. To get a net force you need a field gradient, a field that changes with position. The force is proportional to how fast the field strength changes, not to how strong the field is. This is why the blob only responds within a centimetre or two: close to a small magnet the field falls off extremely steeply, and that steepness is what does the pulling. It also means a bigger, broader magnet is not necessarily better. A sharp-edged magnet with a tight field gradient will pull harder at close range than a large flat one with the same strength, and that is a real prediction your child can test.

The spikes in Step 5 come from the same competition in a different geometry. The material would rather have its particles where the field is strongest, which means reaching up toward the magnet, and gravity and the elasticity of the gel resist. The compromise is a pattern of peaks. Ferrofluids, which use nanoparticles in oil rather than micron particles in gel, do a cleaner version of the same instability.

Two corrections. First, the slime never becomes a magnet. Remanence in fine magnetite is low, so the induced dipoles vanish with the field, which is the collapse you watched. Second, “iron oxide is magnetic” needs qualifying: magnetite, Fe₃O₄, is strongly magnetic, while the common red-brown rust on a gate, mostly hydrated iron(III) oxides, is essentially not. If your child has done our steel wool and rust experiment, test a pinch of that orange rust with the magnet and watch nothing happen. Same element, different compound, completely different magnetic behaviour.

How to Explain It to Your Kid

Ages 5–8: borrowed magnets

This is above the age floor for doing it, but if a younger sibling is watching, the line that works is this: the black dust is full of specks that can borrow magnet powers. When the big magnet comes close, every speck borrows some and becomes a tiny magnet, so they all rush toward it. When the big magnet goes away, they give the powers back and the slime goes floppy again. They should not handle the magnet or the powder.

Ages 9–12: particles, chains and a trigger distance

Introduce suspension and induced magnetism. Then measure. Make three batches with 1, 2 and 3 tablespoons of iron and find the trigger distance for each, three times, and average. More iron means a longer reach. Plot distance against iron added. Then ask the better question: does doubling the iron double the distance? It doesn’t, and working out why gets into how field strength falls with distance.

Ages 13+: dipoles, gradients and yield stress

Give them the real statement: the force on a magnetic dipole is proportional to the gradient of the field, so a uniform field produces torque but no translation. Then the chain-formation picture and the idea of a field-dependent yield stress. The investigation worth running: compare trigger distance for the same magnet lying flat versus standing on edge, which changes the field geometry and therefore the gradient without changing the magnet. Finish with applications, which are real and current: magnetorheological dampers in vehicle suspensions, in prosthetic knees and in building and bridge dampers, all using a field to change a fluid’s stiffness in milliseconds.

The question to ask: “If we could somehow put the slime inside a region where the magnetic field was strong but exactly the same everywhere, what would it do? Think about it before you answer.”

Why Teaching This Early Pays Off

Most primary-school magnetism stops at “magnets attract iron,” which is true and leaves out everything interesting. This experiment introduces three ideas that do a lot of work later: that magnetism can be induced temporarily rather than being a fixed property, that forces on magnetic objects depend on how fast the field changes rather than on the field itself, and that a material’s mechanical properties can be controlled by something external.

That third idea has the longest reach. Smart materials, whose properties switch under a field, a voltage, a temperature or light, are a large part of modern engineering, and a blob of slime that stiffens on command is the most direct demonstration available in a kitchen. Magnetorheological dampers in car suspensions are the commercial example and they work exactly like this.

The measurement in Step 3 is what separates a video from an experiment. Chi and Wylie’s ICAP framework distinguishes watching from doing from generating, and finding a trigger distance three times and averaging is generating. The National Research Council’s Framework for K-12 Science Education names “using mathematics and computational thinking,” and a plot of distance against iron loading is that. What it leaves behind is a physical memory of a field doing work.

Before You Start: What Parents Should Know

Best age	8 and up, with an adult doing the powder and holding the magnet. Not suitable in a house where an under-5 is in the room
Supervision	Constant and hands-on. The adult handles the iron powder, controls the magnet, and puts the magnet away at the end
Real hazards	Magnets first. High-powered neodymium magnets swallowed in twos, or with any other metal object, attract each other through the walls of the gut and cause pressure necrosis, perforation and sepsis. This is documented in the paediatric literature, including a 2007 JAMA report on gastrointestinal injuries from magnet ingestion in US children and numerous case reports of bowel perforation. Use one magnet, make it big enough that it cannot be swallowed, prefer one sealed in a plastic case, and put it away in a drawer the moment you finish. Second, iron oxide powder is a fine dust: an adult adds it slowly, outdoors or by an open window, nobody blows on it, and nobody with asthma leans over it. Third, the magnet will damage phones, bank cards, hard drives and hearing aids, and must be kept away from anyone with a pacemaker or an implanted device. Fourth, two strong magnets snapping together will give a child a blood blister
Don’t	Don’t substitute printer toner for iron oxide. It is a respiratory irritant, it is not meaningfully magnetic, and it stains permanently. Don’t use a set of small magnet spheres or cube magnets, which are the exact products associated with the worst paediatric injuries. And don’t let the slime anywhere near a mouth

When It Doesn’t Work

• The slime is black but barely moves. Not enough iron. One tablespoon colours it without loading it. Go to two or three, kneading thoroughly between additions.
• Nothing happens at any distance. Your magnet is too weak. A fridge magnet or a craft magnet will not do this. You need a neodymium disc of at least 20 mm, and the difference between a weak magnet and a strong one here is total rather than gradual.
• The slime went runny and watery. Iron powder loosens the network and the base was probably already soft. Add a quarter teaspoon more saline and knead.
• It made grey streaks and dry pockets. Under-kneaded. Flatten the slime, sprinkle, fold, and repeat eight to ten times rather than stirring.
• The spikes never formed. Spikes need the magnet within a few millimetres and a well-loaded slime, and they show better on a flattened blob than a ball.
• Everything is black. Expected, and worth planning for. Iron oxide lifts off skin with soap, off hard surfaces with a damp cloth, and off grout and fabric with difficulty. That is why gloves and newspaper are listed as materials.

Take It Further

Measure the gradient, not the magnet. Use the same magnet flat on the tray, then standing on its edge, then with a 2 mm plastic spacer in between. Record trigger distance each time, three repeats. You are changing the field gradient while holding the magnet’s strength constant, which separates “stronger magnet” from “sharper field”.

Build a loading curve. Four batches at 0, 1, 2 and 3 tablespoons of iron. For each, measure the trigger distance and also time how long a 20 g lump takes to creep 2 cm to reach the magnet. Plot both against iron loading. The two curves have different shapes, which is worth arguing about.

What to Watch For Over the Next 3 Months

• Week 4: they start testing household objects with the magnet and sorting them, and discovering that most metals are not magnetic. Aluminium foil and copper coins are the useful surprises.
• Month 2: the phrase “it’s only magnetic while the magnet’s there” or something like it appears, often about a paperclip chain or a fridge door.
• Month 3: worth encouraging: the question of how a magnet can be switched off. The answer is an electromagnet, and building one is the natural next project.

Frequently Asked Questions

Are neodymium magnets safe to have in the house?

A single large one, stored in a drawer and only used under supervision, is manageable from about age 8. Small ones, and especially sets of spheres or cubes, are not: when two are swallowed at different times they attract through the bowel wall and cause perforation, which is documented in the paediatric literature and has led to recalls and warnings. The rule is one magnet, big enough that it cannot be swallowed, put away immediately after use.

Can I use rust from the other experiment instead of iron oxide powder?

No, and finding that out is a genuinely good moment. Common red rust is mostly hydrated iron(III) oxide and is essentially non-magnetic. The black powder you need is magnetite, Fe₃O₄, which has a different crystal structure. Testing a pinch of orange rust with the magnet makes the point in five seconds.

Why does the slime go floppy the instant I move the magnet away?

Because the particles were never magnets in their own right. Fine magnetite has low remanence, so the magnetisation that the external field induced disappears with it. The chains fall apart and the slime is back to being ordinary slime.

Is this the same thing as a ferrofluid?

Related but not the same. A ferrofluid uses nanoparticles coated with a surfactant and suspended in oil, so it stays a liquid and produces much cleaner spike patterns. Yours uses much larger particles in a viscoelastic gel, which makes it stiffen more dramatically but look less tidy.

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About the author

Ricky Flores is the founder of HiWave Makers and an electrical engineer with 15+ years of experience building consumer technology at Apple, Samsung, and Texas Instruments. He writes about how kids learn to build, think, and create in a tech-saturated world. Read more at hiwavemakers.com.

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Sources

1. de Vicente, J., Klingenberg, D. J., & Hidalgo-Alvarez, R. (2011). “Magnetorheological fluids: a review.” Soft Matter, 7, 3701–3710. https://doi.org/10.1039/C0SM01221A
2. Centers for Disease Control and Prevention. (2007). “Gastrointestinal Injuries From Magnet Ingestion in Children—United States, 2003–2006.” JAMA, 297(2), 147. https://doi.org/10.1001/jama.297.2.147
3. Brown, J. C. (2010). “Small Bowel Perforation Caused by Multiple Magnet Ingestion.” The Journal of Emergency Medicine, 39(4), 497–498. https://doi.org/10.1016/j.jemermed.2008.04.007
4. Casassa, E. Z., Sarquis, A. M., & Van Dyke, C. H. (1986). “The gelation of polyvinyl alcohol with borax.” Journal of Chemical Education, 63(1), 57. https://doi.org/10.1021/ed063p57
5. Chi, M. T. H., & Wylie, R. (2014). “The ICAP Framework: Linking Cognitive Engagement to Active Learning Outcomes.” Educational Psychologist, 49(4), 219–243. https://doi.org/10.1080/00461520.2014.965823
6. National Research Council. (2012). A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas. National Academies Press. https://doi.org/10.17226/13165