A joke played on an innocent victim, where several people agree beforehand to laugh at a completely pointless joke involving elephants, polar bears, etc. in a bathtub with the punch line "no soap ... radio". The objective is to see if they can get the victim to laugh along so he won't look like he was too dumb to get it. If he does, then everyone stops laughing and asks the victim what's so funny, and gets a good laugh at his embarassment.
We used to do the old "no soap...radio" joke when we were 10.