Experimental preparation: small paper plate, cotton swab, pigment, milk, detergent. Secondly, put the milk into the paper tray, and I will drop the four colors of powder, blue, yellow and green into the tray in turn. Then dip a cotton swab in the detergent, and finally it's time to witness the miracle. Put the cotton swab vertically in a small paper tray. Wow! We will see a beautiful milk rainbow! What a beautiful milk rainbow! You must be confused when you see this magical scene. Do you want to know how it came into being? Because there is a film on the surface of water to form tension, the tension of each liquid is different. Because the tension of detergent destroys the tension of milk, we can see such a beautiful milk rainbow. Surface tension is a special force, which is a manifestation of the properties of liquid (pure liquid, solution). Microscopically, the surface tension is due to the interaction between molecules in the thin layer of liquid surface, which is different from the interaction between molecules in liquid, thus making the liquid surface layer have a special property. Surface tension is the macroscopic expression of molecular force. Under the action of cohesion, the movement of liquid molecules on the surface always reduces the surface area as much as possible.