Toothpaste....? Not quite.
While your friend is not home get a hold of their tube of toothpaste.
Empty out their toothpaste halfway.
Fill the rest of the tube with a substance of your choice. Chocolate pudding & peanut butter recommended. You don't want to get too gross.
Put a small bit of dab of toothpaste on top of the odd substance so that the tube appears to be filled entirely with toothpaste.
Hear your friend scream as their toothbrush explodes with nasty goop!