The Simple Shape Prediction Trick Effect:
You walk out on stage and place a large card on a chair or easel facing the audience. The card has three shapes drawn on it; a square, a triangle and a circle.
You choose an audience member and ask him to stand. You explain that when people are asked to think of a simple shape, these are the three designs that are normally chosen. You ask him to choose one of the shapes, in this case he chooses the square.
You then ask him to reach under his chair where he'll find an envelope stuck underneath. Ask him to remove the envelope and as he does so, explain that earlier on you made a prediction as to which shape he would choose and stuck it under his chair.
When he opens the envelope, there is a postcard inside that reads 'You will choose the square'. This is a nice opening effect and usually works very effectively.
Make a large card with the three shapes drawn on it as described above. On the back write, “You will choose the triangle”. Write “You will choose the square” on a postcard and seal it in an envelope.
Before the show, stick the envelope underneath a chair in the auditorium.
To perform the effect, simply reveal the appropriate prediction either by asking the spectator to remove the card from under their chair or by asking him to come on stage and turning the card around to reveal the prediction. If he chooses the circle, then your prediction fails but all is not lost.
State that most people choose either the square or the triangle and because he has not done so, that makes him an interesting choice to participate with an experiment.
Note that you don't mention anything about the prediction until the spectator has named their choice.
If your prediction is right, then great! If not, disappointing but you've not yet mentioned your prediction so no one is the wiser. Just get the person up on stage to assist with a sure fire effect whether the prediction works or not.
Its a great way to start your performance off. And you will already have a volunteer to use on stage. Why not try it out for yourself.