This trick has you reading your victim's mind, knowing what number they are going to end up with!
Have your victim start with any number - although warn them that they're going to be doing math with it so it should be reasonably small.
Ask them to multiply that number by itself. Then add their original number to that total.
Now have them divide by their original number. Now add 19.
Finally, have them subtract their original number, and then divide by 4.
Now have them assign a letter to the number with the A=1, B=2, C=3 technique.
Have them think up an animal that begins with that letter.