Sliced Bananas Joke
a sewing needle
It's more of a silly magic trick than a joke, but ...
Before the joke, poke the needle through the skin of the banana on the inside of the curve and right on one of the higher ridges that run the length of the banana.
Swing the needle back and forth crossways inside the banana so that it cuts through the flesh, but not the skin - this is the tricky part, feeling that you've cut through all the flesh.
Repeat this along the length of the banana about every inch. You now have a fancy new pre-sliced banana. The only indication may be a row of little dark spots where the needle was inserted.
The joke is to announce to your group that the grocery store has come out with the coolest thing since sliced bread - sliced bananas! But, these are great because they don't get brown since they're still in their skin.
Prove it to them by having someone open your banana and have all the slices fall right out!