YOU WILL NEED:
a bunch of stuff
Follow the diagram here. ignore the little numbers:
take the weird curvy part on the diagram from the previouse diagram, wich is a wire and run from thirteen, around your doornob, and bak to fourteen.turn it on. when someone opens the door, the wire between thirteen and fourteen will fall off and the alarm will go off. if your little brother or sister is stupid, they won't put the wire back and run away. voila.