"Math Magic" is a wonderful activity to use with students when you find that you have some time to fill!
Ask students to select a three-digit number. However, stress to students that the first and last digits of the number CANNOT be the same (ex. 121, 474, etc.). Once the students have selected their numbers, have them invert the number. For instance, 123 would become 321. Have students subtract the smaller number from the larger of the two, keeping their answer a "secret" from you.
Next, ask a student to give you either the first or the last digit of their answer. You then "guess" their whole answer! How does it work? The center digit will ALWAYS be "9". The sum of the first and last digits will ALWAYS be "9". So, if a student gives you his first number as "2", you instantly know that the answer is 297!! If, however, a student gives you the last number of their answer and it is "9", then you know that their answer is only 99.
ENJOY! The students love this one, and marvel that their teacher is "psychic"! Once they learn the secret, they love to try it out on all of their family and friends!!