Simple Machines: Balance a Lever

Place a pencil on the table.  This is your fulcrum.

Place a ruler on the pencil, so it balances at the 6 inch mark.  This is your lever.

Place 5 pennies on one end of the ruler.  Determine where you need to place another stack of 5 pennies so the ruler balances once again.

Clear the ruler.  Place 2 pennies 3 inches from the pencil.  Where do you need to place a stack of 3 pennies so that the ruler balances once again?

Continue playing with your ruler lever, coming up with as many equations as you can.

Examples, from above...
5 pennies x 6 inches = 5 pennies x 6 inches
2 pennies x 3 inches = 3 pennies x 2 inches.

With enough experimentation (some students will require more than others, some will grasp the idea immediately) students should come to the conclusion that if they multiply the number of pennies by the distance from the pencil, it will need to equal the number of pennies on the other side multiplied by the distance those pennies are from the pencil.