Conclusion

My hypothesis is accepted. After I applied the drop of soap to the milk, the food coloring spread around. Why? At first, the food coloring just sat on the surface of the milk. That's because food coloring is less dense than milk, so it floats on the surface. The milk didn't mix with the food coloring in the beginning because it wasn't stirred together.The soap reduces the surface tension of the milk by dissolving the fat molecules, which is why whole milk works better. The surface of the milk outside the soap drop has a higher surface tension, so it pulls the surface away from that spot. The food coloring moves with the surface, streaming away from the soap drop. Due to the convection that results from the moving surface, the food coloring may be drawn down into the liquid, only to appear rising again somewhere else. When the soap finally gets evenly mixed into the milk, the action stops. If another drop of soap is added, the process starts all over again. This won't work in water because water doesn't contain any fat.

Independent variable: Milk and water
Dependent variable: Whether the food coloring spreads or not