Materials: milk, dish soap, cotton swabs, food coloring, plates
Fats and oils are hydrophobic, but in milk, the fat is just suspended. That means it's mixed throughout, but not really dissolved. And it's not totally "happy" about it. So what would happen if we gave it another option? If it didn't have to be all mixed up with the water?
That's where the dish soap comes in. Dish soap molecules have two parts hooked together--a hydrophobic side and a hydrophilic side. So they'll dissolve in water, but they'll also be attractive to hydrophobic molecules like fats and oils. (That's why it can get those things off your dishes! For a more in-depth explanation, check out this article.) If you mix dish soap with milk, those fat molecules will race to hook up with the hydrophobic ends of the soap molecules. And that race is pretty fun to watch.
Step 2: Put one drop of each food coloring near the center of the plate, but not touching.
Step 3: Cover the end of a cotton swab with dish soap.
Step 4: Dip the cotton swab into the center of your plate, between the drops of food coloring and watch closely!
Step 5: Experiment! You can hold the cotton swab still, or gently twist it back and forth, or dip it on other parts of your plate. You can put the food coloring on different parts of the plate and try to drive them to the center. You can use different kinds of milk (whole, skim, etc.) and see what difference it makes. So beautiful! So fun!