Psy Key by Yves Doumergue

You borrow a few keys. One is selected by a spectator. Mysteriously under impossible conditions, it bends itself in his hands. Once bent, he can't bend it back. Then, you take it with just your fingertips: the key straightens in full view! You immediately hand it back to its owner. 

Psy Key looks and feels like a standard key but is indeed very special. Once you touch it, your body heat makes it bend. You'll either be able to add it secretly to a bunch of borrowed key or switch it with a very common, similar type of key.