ingredients:
ground pepper
cup or bowl of water

(1) cover the water with pepper
Make sure the top of the water is covered with a thin layer of pepper.

(2) let them try to part the pepper
Let someone try to make a path through the pepper. It should be impossible.

(3) your finger "magically" makes a path
Now it's your turn. The pepper seems to fear your touch, parting where ever you put your finger. How did you do this?

(*) the secret weapon
Soap! Before starting the trick, put a little soap on your finger. Dish soap works good.