(You can click on the picture to get a clearer shot of our game.)
With a marker, write the numbers 1-12 in the first egg carton. Then 1-6 on each side of the second egg carton. In the second carton put the correct number of beans in each side, using two different kinds of beans (we used pintos and garbanzos, but you could also use 2 colors of beads, or marbles, or what ever). That's it. Now play!
One way to play: take the beans from a #2 slot and put it in the #4 slot. How many of the other kind of bean do you need to make 4?
Obviously you can do whatever numbers you want to do. You can add, subtract, work forward, work backward... all kinds of math play is possible with this practically free game!