Find a cookie sheet or other shallow dish and set it on the table. This experiment works best with a shallow dish.
Pour into the cookie sheet room temperature milk, enough that you cannot see the bottom. If you get the milk directly from the refrigerator then you can microwave the milk to room temperature. Make sure your preschooler can see the milk easily in the cookie sheet.
Squirt the food coloring around various spots of the milk. It's best to keep the colors close for this experiment.
Take a toothpick and dip it into the dish soap and touch it to the milk on the sections with the food coloring. Hold it there for a few seconds, approximately 5 to 10.
Remove the toothpick and notice the continuous swirls going on even without the toothpick.
Dip the toothpick in the dish soap again and then touch the milk to continue the swirls.