The Trick
1. Select 2 volunteers.
2. Place a pencil and a paper clip on the desk.
3. Turn your back and close your eyes (or blindfold yourself).
4. Instruct each of the volunteers to pick up one of the items on the desk.
Assign the number 7 to the person who took the pencil.
Assign the number 9 to the person who took the paper clip.
5. Have the first student multiply her number by 2.
Have the second student multiply his number by 3.
6. Now have them tell you the SUM of these products.
7. If the sum is divisible by 3, the second person took the pencil.
If the SUM is not divisible by 3, the first person took the pencil.
Here is an example:
Suppose the first volunteer selects the paper clip and the second person selects the pencil.
The first person multiplies 9 x 2 = 18.
The second person muliplies 7 x 3 = 21.
The sum is 39, which is divisible by 3, so you know that the second person took the pencil.