Table of Contents
Why Oil and Water Finally Mix: The Mayonnaise Test
An emulsion experiment for kids in three jars: oil and water, oil and soap, and real mayonnaise. Why it breaks, how to rescue it, and the egg safety rule.
Shake oil and water hard enough and they mix for about four seconds. Then they split, every time, no matter how hard you shook. Add one teaspoon of something and they stay mixed for months. That one teaspoon is the point of this emulsion experiment for kids, and the best version is not a demonstration at all. It is mayonnaise. You will turn 180 millilitres of oil into a thick white sauce using one egg yolk, and if you pour the oil in too fast you will watch it fail and be able to explain exactly why.
Key Takeaways
- Three jars, 20 minutes: oil and water alone, oil and water with dish soap, and oil and water with egg yolk, which becomes mayonnaise.
- The concept is emulsification: molecules that stick to both oil and water sit at the boundary and stop droplets merging.
- Good for ages 8 to 14. The whisking is hard work, which is part of the lesson.
- The classic failure: adding oil too fast. The mixture goes thin and greasy instead of thick, and it is rescuable.
- You can eat the mayonnaise only if you use pasteurised egg. Never eat the soap jar.
What You’ll Need
| Item | How much | Where to get it | If you don’t have it |
|---|---|---|---|
| Neutral oil (sunflower, canola) | 180 mL plus 30 mL for the jars | Supermarket | Light olive oil; extra virgin turns bitter when emulsified |
| Pasteurised egg yolk (contains egg) | 1 | Supermarket; sold as pasteurised shell eggs or liquid yolk | Aquafaba, 45 mL of chickpea tin liquid, for an egg-free version that works |
| Dijon or yellow mustard | 5 mL | Supermarket | Skip it, but the mayonnaise is less stable |
| Lemon juice or white wine vinegar | 15 mL | Kitchen | Either works; vinegar is sharper |
| Salt | 1 g | Pantry | Any salt |
| Dish soap | 2 drops | Sink | Any washing-up liquid |
| Small jars with lids | 2 | Kitchen | Glasses with cling film over the top |
| Mixing bowl and whisk | 1 of each | Kitchen | A fork and determination, or an electric mixer |
| Food colouring | 1 drop, optional | Baking aisle | Makes the water layer visible in the jars |
Time: prep 5 min, experiment 20 min, cleanup 10 min Cost: everything is probably in your kitchen except pasteurised eggs Mess level: medium. Oil on a worktop is worse than it looks.
Do the Experiment
Step 1 — Jar one, the control
Put 30 mL of water and 15 mL of oil in a jar, add a drop of food colouring to the water first if you have it, seal it and shake for 20 seconds. Set it on the table and watch. Ask your kid to time how long until two clean layers are back.
Step 2 — Jar two, add soap
Same amounts in the second jar, plus two drops of dish soap. Shake for 20 seconds. Set it next to jar one. Label this jar clearly, because it looks like something you could taste and must not be tasted.
Step 3 — Start the mayonnaise base
In the bowl, whisk together the pasteurised egg yolk, the mustard, the lemon juice and the salt until it is smooth and slightly paler. Thirty seconds is enough. This is the only part where the emulsifier is on its own, and it should look like thin custard.
Step 4 — Add the oil drop by drop
This is the step that decides everything. Add the first 60 mL of oil literally a few drops at a time, whisking hard between each addition. The mixture should thicken and lighten as you go. Only once it is visibly thick can you move to a thin, steady stream for the remaining 120 mL.
Step 5 — Compare all three
Put the finished mayonnaise next to the two jars. You now have oil and water separated, oil and water held together by something inedible, and oil and water held together by something that came out of an egg. Same physics in all three.
What you should see: Jar one separates into two sharp layers within 10 to 60 seconds, and the boundary is a clean line. Jar two stays uniformly cloudy and pale for hours, sometimes days, with no visible line. The mayonnaise thickens noticeably somewhere around the 40 mL mark and ends up pale, glossy and stiff enough to hold a peak. Total whisking time is six to ten minutes by hand, and your arm will know it. If you rush the oil, the mixture stays thin and then suddenly goes greasy and curdled, which is called breaking.
What’s Actually Going On in This Emulsion Experiment for Kids
Oil and water separate because of what each molecule wants. Water molecules hydrogen-bond to each other strongly. Oil molecules are long hydrocarbon chains that cannot hydrogen-bond at all. Putting them in contact costs energy, so the system minimises contact area, and the smallest possible area between a given volume of oil and a given volume of water is one flat boundary. Shaking makes millions of tiny droplets, which is a huge amount of boundary. The droplets then collide, merge to reduce boundary, and you get your two layers back.
An emulsifier is a molecule with a water-loving end and an oil-loving end. It cannot be anywhere comfortable except exactly at the boundary, so it crowds onto the surface of every droplet. Two things follow. The interfacial tension drops, so making droplets costs less energy. More importantly, the coated droplets cannot merge, because merging would mean pushing the emulsifier layers out of the way. McClements’s standard text on food emulsions makes the key point that follows from this: emulsions are not thermodynamically stable, only kinetically stable. Mayonnaise will separate eventually. It just takes months instead of seconds.
Now the part most explanations get wrong. The usual story is that lecithin in egg yolk is the emulsifier. Lecithin, meaning the phospholipid fraction, does contribute, but Marc Anton’s 2013 review of egg yolk structure and function in the Journal of the Science of Food and Agriculture describes the low-density lipoprotein fraction as the main driver of yolk’s emulsifying capacity. Those are lipoprotein particles that spread apart at the interface and leave behind a protein and phospholipid film. Kiosseoglou and Sherman had shown the same thing experimentally in 1983. So the honest version is that egg yolk is a cocktail of emulsifiers, and the lipoproteins matter more than the lecithin.
The numbers are worth stating because they are surprising. Under US regulation, 21 CFR 169.140, mayonnaise must contain at least 65 percent vegetable oil by weight, with an egg-yolk-containing ingredient and an acidifier such as vinegar or lemon juice. Most mayonnaise is closer to 75 percent oil. And yet water is the continuous phase: mayonnaise is oil-in-water, with oil droplets a few micrometres across packed so tightly into a small amount of water that the whole thing behaves like a solid. That packing, not thickener, is why it holds a peak.
The breaking failure is the mechanism showing its limits. If you add oil faster than the whisk can break it into droplets, you get large volumes of oil arriving at once, with nowhere near enough emulsifier coverage to go around. The oil droplets coalesce, the structure inverts, and you end up with water droplets in oil instead. That is why the rescue works the way it does: start fresh emulsifier in a clean bowl and feed the broken mixture in slowly, so every bit of oil meets enough yolk.
How to Explain It to Your Kid
Ages 5-8: Molecules that hold hands with both
Tell them water molecules hold hands with other water molecules, and oil molecules hold hands with other oil molecules, and neither will hold hands with the other kind. So they push apart. Then there is a special molecule that has one hand for water and one hand for oil, so it can stand in the middle. Have them shake jar one and jar two themselves and watch the difference. Three-year-old vocabulary, correct physics.
Ages 9-12: Surface area and droplets
Introduce droplets and surface. Shaking makes millions of small droplets, which means a lot of boundary, and the system wants less boundary, so droplets merge. An emulsifier coats each droplet so they cannot merge. Then give them the mayonnaise job and let them feel the moment it thickens. Ask them to describe, in their own words, what changed in the bowl at that moment.
Ages 13+: Kinetic versus thermodynamic stability
This is a genuinely useful distinction and this is a good place to learn it. Mixing oil and water is unfavourable, and no emulsifier changes that. What an emulsifier changes is the rate: it raises the barrier to coalescence so high that separation takes months. Have them deliberately break a small batch by pouring oil in fast, then rescue it, then write down why the rescue works. Then ask what would happen if they doubled the oil with the same single yolk.
The question to ask: “If soap and egg yolk do the same job, why does the dishwasher use soap and not egg yolk?”
Why Teaching This Early Pays Off
Emulsions are everywhere a child already looks. Milk, butter, ice cream, salad dressing, paint, moisturiser, sunscreen, bitumen on the road. The underlying idea, that you can force two incompatible things to coexist by putting a go-between at the boundary, is also the idea behind detergents, cell membranes and a lot of industrial chemistry. A kid who has made mayonnaise has done surface chemistry with their arms.
There is a second payoff that has nothing to do with chemistry. This experiment fails in a specific, diagnosable, recoverable way, and that combination is rare. Kirschner, Sweller and Clark argued in 2006 that leaving learners to discover things with minimal guidance works poorly, precisely because they do not know which features of the situation matter. Here, the feature that matters is the rate of oil addition, and you can tell them in one sentence. Then they break it, fix it, and own the explanation. That is guided practice, not unassisted discovery, and the difference is the whole reason it works.
Keep expectations proportionate. Making mayonnaise once will not improve anyone’s chemistry grade. It will give them a physical referent for the word “emulsion” and a story about a failure they fixed, which is a better foundation than a definition. It also pairs naturally with the experiment on why soap beats water on grease.
Before You Start: What Parents Should Know
| Best age | 8 to 14. Under 8, the whisking is too much work and the drop-by-drop patience is unrealistic. |
| Supervision | Close for the egg handling and the labelling of the soap jar. Kids can do all the whisking. |
| Real hazards | Raw egg carries a risk of Salmonella. USDA guidance is that shell eggs may contain Salmonella Enteritidis and must be handled carefully; pasteurised eggs have been treated to destroy it. Contains egg, and mustard is itself an allergen for some children. Spilled oil makes a floor slippery. |
| Don’t | Do not use an ordinary unpasteurised egg and then eat the result, and never let anyone taste the soap jar. Keep the two jars visibly different, with a label. |
On eating it: the mayonnaise is food, and good food, on one condition. Use pasteurised egg, either pasteurised shell eggs or pasteurised liquid yolk, both sold in larger supermarkets. If you cannot get pasteurised egg, make the aquafaba version instead, which uses the liquid from a tin of chickpeas and emulsifies surprisingly well. Refrigerate immediately and eat within 24 hours. The soap jar is never food and should go down the sink at the end.
When It Doesn’t Work
- It broke: thin, greasy and curdled. Oil went in too fast. Put a fresh pasteurised yolk, or a tablespoon of water, in a clean bowl and whisk the broken mixture into it a teaspoon at a time. It comes back almost every time.
- It never thickened at all. Either the oil went in too fast from the very start, or the yolk was too cold. Room-temperature ingredients emulsify far more readily than fridge-cold ones.
- It is thick but tastes bitter. Extra virgin olive oil. Emulsifying breaks down its structure and releases bitter polyphenols. Use a neutral oil, or no more than a quarter olive oil.
- The soap jar separated after an hour. Not enough soap, or you used a very dilute brand. Two full drops in 45 mL of liquid is the target, and the mixture should look milky rather than faintly cloudy.
- It split in the fridge overnight. Normal, and the point about kinetic stability. Whisk it briefly and it comes back. Commercial mayonnaise resists this because it is made with a homogeniser that produces far smaller, more uniform droplets.
Take It Further
Find the minimum emulsifier. Make four small batches with a quarter yolk, half a yolk, one yolk and two yolks, each with a fixed 100 mL of oil. Record which ones hold and which break, and how long the successful ones survive in the fridge. You are measuring how much interface one yolk can cover, which is a real physical quantity.
Test emulsifiers against each other. Run egg yolk, mustard alone, honey, aquafaba and soy lecithin granules on identical oil-and-water jars. Shake each for exactly 20 seconds and record the time until a visible layer reappears. Mustard on its own is surprisingly effective, which is why it is in almost every vinaigrette recipe, and the comparison connects neatly to shaking cream into butter, where the goal is to break an emulsion rather than build one.
What to Watch For Over the Next 3 Months
- Week 4: They shake a bottle of salad dressing and comment on how long it stays mixed, unprompted.
- Month 2: The word “emulsifier” shows up when they read an ingredients label. Lecithin in chocolate is the usual first sighting.
- Month 3: They predict a result in a new situation, such as whether a sauce will split, and get the reasoning right even if the answer is wrong. The reasoning is what matters.
Frequently Asked Questions
Is homemade mayonnaise safe for children to eat?
Only if you use pasteurised egg. US Department of Agriculture guidance is that raw shell eggs can carry Salmonella Enteritidis, so pasteurised shell eggs or pasteurised liquid yolk are the safe route for anything containing uncooked egg. Refrigerate and use within a day.
Why does my mayonnaise keep breaking?
Almost always because the oil went in too quickly at the beginning. The first third of the oil needs to go in a few drops at a time. Cold ingredients and a bowl that is too wide for the volume also make it harder.
Can we make it without egg?
Yes. Aquafaba, the liquid from a tin of chickpeas, contains proteins and saponins that emulsify well, and research on its technological properties has documented that capacity. Use 45 mL in place of one yolk and add the oil the same way.
Why is the soap jar not food?
Dish soap is a synthetic surfactant designed to be rinsed away, not eaten, and it irritates the gut. It does the same physics as egg yolk, which is exactly the interesting part, but the molecules are different and are not food-grade.
Does it have to be whisked by hand?
No, and an electric mixer makes it nearly foolproof because it breaks droplets faster than you can pour. Hand whisking is better for the lesson, because the moment the mixture thickens is something you feel in your wrist before you see it.
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.
Sources
- Anton, M. (2013). “Egg yolk: structures, functionalities and processes.” Journal of the Science of Food and Agriculture, 93(12), 2871-2880. https://doi.org/10.1002/jsfa.6247
- Kiosseoglou, V., & Sherman, P. (1983). “The influence of egg yolk lipoproteins on the rheology and stability of O/W emulsions.” Colloid and Polymer Science, 261(6), 502-507. https://doi.org/10.1007/bf01419834
- McClements, D. J. (2015). Food Emulsions: Principles, Practices, and Techniques, 3rd edition. CRC Press. https://doi.org/10.1201/b18868
- U.S. Food and Drug Administration. 21 CFR 169.140, “Mayonnaise.” Code of Federal Regulations. https://www.ecfr.gov/current/title-21/chapter-I/subchapter-B/part-169/subpart-B/section-169.140
- U.S. Department of Agriculture, Food Safety and Inspection Service. “Shell Eggs from Farm to Table.” https://www.fsis.usda.gov/food-safety/safe-food-handling-and-preparation/eggs/shell-eggs-farm-table
- Tufaro, D., & Cappa, C. (2023). “Chickpea cooking water (aquafaba): technological properties and application in a model food system.” Food Hydrocolloids, 136, 108231. https://doi.org/10.1016/j.foodhyd.2022.108231
- Kirschner, P. A., Sweller, J., & Clark, R. E. (2006). “Why minimal guidance during instruction does not work.” Educational Psychologist, 41(2), 75-86. https://doi.org/10.1207/s15326985ep4102_1
