Preparation: 

Using sticky tape stick some pencil led under your thumb nail.

Showtime: 

Give a spectator a notepad and ask them to write any number on their. You pretend to write something with a pencil on your notepad. When they have done ask them to reveal their number. As soon as you see their number secretly write it using the pencil led under your thumb on your notepad. Then reveal what you wrote and accept your applause.

P.S 

You could ask them to think of a shape, any random drawing or just about anything.