I do this way too much at school, mostly when people take too long in the toilets.
What you need
Toilets
Friends
Hand dryer

What to do
After you've been to the toilet and you're waiting on your friends, start up a conversation. It must be something they'll both like so once they start talking you drop out. Then pretend to dry your already washed hands.

At first they'll just think your drying your hands and carry on talking but every time they start again, you start again as well. Because the hand dryer makes a pretty big sound they'll have to stop each time. As soon as one of them come out, sprint away or your face will be the wet and their hands dry. (Not joking. They actually do that to me.)

After doing this, try avoiding every toilet trip with them as they will get you back...😳