Always get the Number 99 trick
This is a nice mind twisting trick
As you will see it does depend on certain specifics. There are certain instances where this trick wont work. Most of the time this trick will work. Experiment with it and see for yourself.
You can use a volunteer or try it yourself.

Write down any two different numbers.
Example 29

Then reverse the two numbers.
Example 29 becomes 92

Subtract the smaller number from the larger one.
Example 92 – 29 = 63

Take the result and reverse the digits
Example 36

and add that number to the answer you got when you subtracted the numbers.
Example 36 + 63 = 99