Hi Guys,
What stage trick could I perform with the whole audience trying to do it. Which then turns out to be a real fooler with a surprise at the end! Get your thinking heads on! Could expose part of the trick?

Of course there is always the turn your hands right side up gag. Copperfield uses it and it has been discussed here many times. Try to find the Magic For Dummies book. It's in there. Hope this helps and I am sure many others here have some good ideas for you.

There are also the numerous mathematical tricks a-la Max Maven.