Want to make them laugh?
They think they know what's going to happen- but they get a surprise!
Place two red sponge balls in your hand. Make sure to do it in an obvious manner, so that there is no question that the spongers are in that hand. You may even choose to have one of the sponges "accidentally" peek out of your hand, as if you don't notice. Ask your spectator which hand the balls are in. He will correctly name which hand (you've made it obvious!). So you tell him, "Well, I guess you've caught me red-handed!". When you open your hand, both sponge balls are gone- and instead, you are holding a Big Red Hand!