Pepper (or any small particle that will float on the water)
This one is an oldie but a goody. I think this is the first science experiment I ever did!
1. Get a bowl and fill it half way or so with water.
2. Shake pepper over it until the pepper covers most of the surface of the water.
3. Now take your finger and put some dish soap on it.
4. Dip your finger into the water and watch the pepper scatter to the side of the bowl!
5. You will need to empty bowl to do it again.
The reason for this is surface tension. Water has a very thin “skin” on it (it’s not really a skin, it’s a quality of the molecules of the water at the surface, but it has similar properties in some ways). The dish soap destroys the “skin” bursting it like a bubble. As the “skin” retreats from the dish soap, it takes the pepper with it.