Glass of water
A paper cup
Place a bowl containing water on the table.
Sprinkle the ground pepper into the bowl, such that it forms a thin layer on the surface of the water.
Take a toothpick and dip the tip in the dishwashing liquid.
Now, touch the tip of the toothpick in the water bowl, in the centre.
You will notice pepper particles moving away from the center and spreading out.
How does it work?
This happens due to ‘surface tension’. Surface tension is the property of a liquid to resist external force. Pepper floats on water due to this. But, when the dishwashing liquid comes in contact with water, it destroys the surface tension around it, allowing the particles on the surface to spread. This is the reason why soaps are used to clean items.