This is a revenge prank that will hurt a little. Note that you may not get the most satisfying result if the victim has glasses on.

Step 1: What You Need!

First of all you will need a victim with a hand!

Step 2: Face-palm

Tell the victim to put their hand on their face like so: ( with their index finger on the center of their forehead )

Step 3: Thwap Time!!!

With their hand still on their face, tell them to resist your pull. Take their index finger and pull it (it should be a little hard to pull it since they are resisting) Then let go and--THWAP!!!! ( sorry, the picture I have is using the middle finger, either way works, but index finger is better! )