How does soap work?  What do hydrophilic and hydrophobic mean? This is a good experiment to start some of these discussions.  The fats in the milk are oil based (technically, they're called lipids) and the water in the milk is well, water.   Fats and oils are hydrophobic, which translates into "water fearing".  These fat molecules will clump together so that as few of them touch the water as possible.

Soap is a molecule that has a hydrophobic side and a hydrophilic side. (Hydrophillic ==  water loving. This is useful in our daily lives because the hydrophobic side of soap molecules will bind to oil and grease, and the hydrophilic side of the soap molecule will bind to the water molecules.  Then, when you wash your hands, as the soap gets washed away, the oils and grease are dragged away along with the soap.  

So, back to the experiment.

When you add soap,  you'll see food coloring expand away as all the fat molecules push the food coloring out of the way to get closer to the hydrophobic soap molecules.   Kids of all ages will enjoy "painting" with soap in the milk.  Older kids can start to grasp the concept of hydrophobic vs hydrophilic.

Materials:

Plate

Milk (skim milk works best, but whole milk will also work)

Half and half

Soap [not all soaps work.  Joy dish soap does]

Q-tips or Toothpicks

Food coloring

What I do:

I pour milk into one of the plates. Then, we drop a few drips of food coloring around the plate.  The kids dip their Q-tips into the soap and then poke one of the food coloring drops.  The result should be to see the food coloring expand away from the soap.

To make this an experiment, I get another plate and pour in half-and-half (or a milk with a higher fat content than the one I previously used.).  Before the kids dip their soap into the new plate, I ask them whether the colors will spread more or less than last time, and I ask them why.  This is called 'making a hypothesis' and is crucial to running an experiment.   Then I let them test it out.

What this activity teaches:

Scientific Method.  It's important to have your kids guess (make a hypothesis) about what will happen.  To encourage more experimentation,  ask them what they think would make the colors expand faster?  I.e does warm milk vs cold milk work?  What about adding more dish soap vs adding a little bit?  See what they come up with first before you give them hints.

Hydrophobicity.   Certain materials repel water.  This is a great experiment to start the conversation of what hydrophobicity is, and how it is useful in everyday life.