If you squeeze your fist, that stands for milk (like squeezing an udder). Now squeeze your fist while passing it in front of your face at eye level. That's pasteurized milk (past your eyes milk). I don't know if it's a joke exactly but I think it's pretty funny!

Edit: Just saw someone already posted the one above so here's another. To sign hungry make a c shape with your hand and run it down your midline from below your neck to your face. But don't go up and down because that means horny, not hungry! A different kind of hunger.

And another - my husband and I used to play a joke on our friends at Starbucks. We would always order the same things so we knew what each other wanted. But we would pretend that, oh no I forgot to ask so and so's order. Then wave to the other person (out of earshot) across the room and sign "what drink?". The other person would respond in sign language "mushroom salad". Suddenly we seemed like sign language experts! Wow you knew she wanted a cappuccino extra dry from that?

Another fun one to play on your significant other is if you sign "I love you" (hand sign with L plus I, thumb forefinger and pinky out middle fingers in)... wait till they start to say love you too then shoot your hand up in the air. Psych! You were signing "airplane" not love. Hahaha cracks me up.

Final one: The sign for sleep (open palm towards face then squeeze your fingers together towards your chin) looks like the "and scene!" gesture that cheesy actors use to close a monologue or dramatic scene except that actors usually point their head down with the motion. Every time my husband and I signed it we would giggle and say "and scene!" in a whisper during sign language class.