You hold a bottle in your hands.
Show the audience the cap from the bottle.
Put the cap on the bottle with an abrupt gesture, and everyone hears the cap fall inside.
Right through the narrow neck of the bottle.
The secret of the magic trick lies in the bottle having a slit that’s covered by the opaque label. And in the presence of a second bottle cap, which had been previously inserted into the bottle through the slit. At the start of the trick, when you show the bottle to the audience, you discreetly hide the cap at the bottom of the bottle with your hand. When you hit the bottle with the other cap, the one inside bounces around, giving the impression that it has just fallen into the bottle. The second cap, of course, remains unnoticed in your hand and you drop it into your lap or a pocket when everyone is distracted by the cap in the bottle.