To understand the property of surface tension.
What will happen if a coin is added to a glass brimful with water? Will water spill out? Try it yourself.
a glass of water, few coins
Fill the glass with water up to the brim. Hold a coin vertically and gently drop it into the glass. Does water spill out? What will happen if one more coin is added?
Add 2nd, 3rd, 4th… coin until water finally overflows. To your surprise, a glass brimful with water can accommodate as many as 15–20 coins.
After adding a few coins observe the water surface. You will find water bulging out at the surface.
Water molecules at the surface strongly hold one another and behave like a `water film' stretched across the surface. This property of a liquid is called surface tension. The cohesive forces together with the surface film prevent water from spilling over.
When water bulges too much, it becomes difficult for the film to hold the water any more, so it spills out.
When soap is added the surface tension of water is reduced, because the cohesive forces between the molecules is reduced. Try this exercise with soap solution. How many coins can be put in a glass with soap solution?