Best joke to play on New FAs is to call them up in flight (towards the end) and tell them that the company just sent you a message stating that the apple juice on board may be contaminated. They need you to pour 8 ounces in a glass and it needs to be handed to the gate agent as soon as the cabin door is opened so it can be sent to the "airport medical lab?" (or make up some big words up with lab at the end) for testing. Gate agent will think it's urine and you'll get a good laugh. Works every time as long as they are all new.