I thought the balance scale would be a great way to demonstrate how simple equations work, and what it means to solve for an unknown. I needed something to hide the unknown quantity under, and an empty plastic Easter Egg met the requirement nicely. I placed the empty egg half on one side of the balance and calibrated the scale (there are sliding weights on the top bar to do this with) so it balanced in the middle. Then I hid 3 gram cubes under the egg, added 5 more to that side and 8 to the other—and showed the children that the two sides of the scale balanced out.
I asked what that meant—at first they thought the egg shell was adding the extra weight, but I explained that I had adjusted the scale to balance with the egg shell (I should have showed them this before I started). Then I told them there were cubes under the shell and asked them to figure out how many there were. Honeybee figured out that there must be 3, and we lifted the shell to check. Sure enough ,there were three cubes hiding underneath, making 8 on each side—which was why the scale balanced. I then showed them how to write the equation on paper, using an x for the unknown.