To the untrained eye, this key looks completely normal. Challenge members of the audience to get the key into the bottle. No matter how hard they try, they won't be able to do it. But with a tissue or cloth covering, you can put into the bottle smoothly, because the key folds!
The key can also be used for other tricks like "key through ring" where the key penetrates through a ring while it is wrapped.
Each pack contains two keys: one is normal key, and the other is a special folding key that you switch in and out to perform the effects.