Overhead Projector Trick

I found that this trick works well on an overhead projector.
It is one of my favorite tricks because it involves two variables.
Place the followiing four items on the projector:
(1) Pencil
(2) Key
(3) Stamp
(4) Yarn

Now close your eyes and give the following instructions:
1. Have a student point to one of the four items.
2. Have the students count the number of letters in that word.
3. Multiply by five.
4. Add three.
5. Double.
6. Have someone hold up a number of fingers from 1 to 9.
7. Add the number of fingers to the previous sum.
8. Ask what number the students have. ..
..
..
..
To determine the object chosen and the number of fingers held up,
take the number they give you and subtract six.
The tens digit will be the number of letters in the object and
the units digit will be the number of fingers held up.

Why does this work?

Let's use algebra to explain it.

2. Have the students count the number of letters in that word. Let x = the number of letters in the object chosen.

3. Multiply by five. Then we get 5x

4. Add three. We have 5x + 3

5. Double. Now we get 10x + 6

6. Have someone hold up a number of fingers from 1 to 9. Let f = the number of fingers

7. Add the number of fingers to the previous sum. We obtain 10x + 6 + f

When you subtract 6 from the expression 10x + 6 + f,
you obtain 10x + f.
The number of letters in the object has been multiplied by 10
and is therefore in the tens place, leaving the number of fingers in the units place.