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!
This one has lots of comedy potential! If kids are yelling out "I saw that!" or "I know how that's done!" and you want to calm them down, let them try and see which hand the red sponge is in. Whatever they say, you can say, "No, the red sponge is not there- but I guess you caught me red-handed!" as you produce the sponge hand.Or, at the end of a trick where you have used a volunteer, do this trick, producing the hand and saying "Let's give our volunteer a hand!".