Hidden inside an envelope is an apparently very animated group of flies!
Secret: This special prop will really create a buzz! Inside the envelope is a secret mechanism, with a metal ring attached to it via an elastic band. If you wind up the ring (approximately ten turns) and then carefully replace the mechanism in the envelope, anyone trying to remove the contents will be in for a shock.
When the envelope is opened and the mechanism pulled out, the ring will spin causing a loud noise and a vibration. Try it – even when you know what is about to happen you will still jump!