Experiment with the milk and toothpick. Suggest a hypothesis that might explain the phenomena that you observe. The most important aspect of this activity iare the observations, hypotheses, and conclusions that they students draw. Whether or not they come up with the right answer is not important. Although the phenomena appears to be related to the detergent action on the milk, it has not be proven what causes this activity to occur.
Milk is more a colloid than a simple solution.
It contains not only salts and sugars dissolved in water, but also small globules of fatty substances and protein which vary in diameter. The fat globules, being hydrophobic , cannot dissolve in the water. They can, however, dissolve in each other. It will reduce the surface tension of water. The detergent tries to surround the fat in the milk but the fat is so evenly dispersed that is simply turns over and over.