There's an old, but neat trick with balloons: stick a tiny piece of clear sellotape on a balloon, then find someone sensitive and jumpy and tell them you're going to pop the balloon. Brandish a pin, and with a mighty flourish jab it into the balloon - but jab it through the piece of sellotape. The balloon won't burst; you can even pull the pin out and it won't lose pressure very fast.

Great trick for getting a rise out of people who probably deserve better