Rattlesnake Eggs:

Tell your friends you just got back from a trip to the desert and you brought them back a souvenir. Watch them open the bag and jump! A wound up elastic inside releases then the bag is open and creates a rattle...gets them every time. Wind-up the rubber band and washer. Place it in the bag. When an unsuspecting person opens it, they get a scare. Expecting to see eggs, they hear a rattlesnake noise instead.