pepper away
Sprinkle pepper on the surface of water in a bowl. Most of the
pepper will stay on the surface of the water. Touch the water
with the end of a toothpick and not much happens. Put a tiny
amount of dish detergent on the end of the toothpick and touch
it to the center of the water's surface. The pepper rushes away
from the toothpick and sinks. Use talcum powder or paprika
instead of pepper and the same thing happens.
The water molecules at the water's surface pull on each other.
That causes the molecules at the surface to act somewhat like
a very thin sheet of rubber stretched across the surface. This
is known as surface tension. The pull is weakened by the
detergent at the center where the toothpick is placed. The
stronger pull around the edges brings the pepper toward the
sides of the bowl. As the detergent spreads out it destroys the
surface tension and the pepper sinks.