This is a really simple mental magic trick that really works on math principles and should never really fail.
Ask a member of the audience to think of a number between one and 10.
Get them to double it then to add four and then to divide by two and to take away the number which they first thought of. You can easily predict the number which they will end up with which should always be number two.
To give an example: if they think of 7, after doubling its they will have 14 and when they add 4 it will make 18.
Finally when they divide by 2 and take away the first number they thought of (7), they will end up with the number 2!