This is a fun game you can play with your students and they must figure out how the "magic" works. This might work very well with pre-algebra students or beginning algebra students.
Tell students to pick a number 1-9. From here you can tell them to do what ever you want to with the number (add 5, multiply by 2, subtract 10 etc.) Don't get too carried away because the trick won't work if the number is too big (cannot be greater than 60).
The last two steps they must do is to multiply by 9 then add the digits together. The result will always be 9. Then you magically tell them that their result is 9 and they will be completely confused. You can further trick them by adding a few steps after they add their digits together then tell them what their result is.
Do it again and again until someone figures out the trick. Its a lot of fun!