A Cool Mathematical Magic Trick
Have your volunteer (or even an entire audience) pick a planet. They are then to count clockwise the number of letters in their planet’s name. (For convenience, that number is in parenthesis.) From their new location, they again count the number of planets that match the number of letters in their new planet’s name. This time, they will always land on Earth. If they do it again, they will land on Mars.
After their third move, announce they have landed on Earth and they will be amazed. (Or, wait til after their fourth move and announce they are on Mars.)
Naturally, they should be doing their counting and moves in their head, or behind your back, so that it appears that you would have no way of knowing their location.
To make this more amazing, you could have them spell the names of the planets (in their head) and move one space for each letter.

For more cool mathematical magic schedule Dennis Regling to do his math show at your school.