Share anecdotes. Here's a mild one to get you going:
I used to quite regularly wait silently outside the bathroom door with my face pressed right up against it and my hands up in the air doing a scary face, so when the person opened the door to come out they screamed in terror.
Discuss the ethics:
I used to live with two guys, one of whom loved to do practical jokes, the other one being vocally and passionately opposed to them, and very easily wound up by them. Guy B was really asking for it with his reaction, and Guy A was completely unrelenting in his zeal. Guy A's favourite was leaving a full plastic cup of water on top of Guy B's bedroom door so it fell on him when he opened the door. He also once "borrowed" his phone and sent his Mum, Dad and sister a coming out text. Loads of other stuff I can't remember, which basically destroyed their friendship beyond the point of recovery.