Knock Knock Door Chime

To both confuse and punish your visitors, the Knock Knock Door Chime rings a door chime whenever someone knocks. Wha? Yes, you read that right.

One end of the wireless door chime attaches to the inside of the door, while the other attaches anywhere up to 100 feet away in your house. When someone knocks, the "knock detector" converts the knock into a chime, which sounds the chime on the other unit. Simple, yet confusing to a guest when he hears a doorbell whenever he knocks—especially after spending two minutes looking for the doorbell.

This new-age retardetry will cost you $49.95.

Knock-Knock Door Chime [Gizmo City via Red Ferret]