Ask each guest to draw a square. The ask them to draw four lines diving the square into nine small squares. Now each one is to put a number in each square, using the number from 1 to 9. No number can be used more than once. It should be explained that the number must be placed so that they total 15 anyway they are added — horizontally, vertically or diagonally.
Answer: The secret lies in placing 5 at the centre and the working the corners with the numbers 2, 4, 8 and 6. See who can work it out first.
What's Your Number?
Take a number. Double it. Add 9. Subtract 3. Divide by 2. Subtract the number you first had in your mind. You can tell them that the answer is 3. It will be invariably be 3.
Another example: Suggest that someone take a number. Double it. Add 12. Subtract 4. Divide by 2. Subtract the secret number. You announce that the resulting number is 4.
The answer in each case will always be the subtracted number.