This idea popped into my head the other day when trying to think of something fun to do with cereal boxes. I always loved marble runs when I was a kid, and this one was very easy to make. Jack and Tess really loved it too. We had to go finish off some more cereal so I could make one for each of them.
To make one, first tape the lid of the box shut, and then using a knife, cut off the front panel. Leave a little ledge on the bottom to keep the marbles from rolling out everywhere. Then take the detached front panel and cut it into three strips. Bend them at the ends and cut a notch on one side for the marble to run through. Next I took a little extra cardboard from another cereal box and folded the end of it and taped it onto the bottom (see photos). The folded end makes the marble run tilt back a little bit so the marble goes down the holes and doesn’t fall out of the box. Finally, tape the strips into the box and cut a little hole at the top to drop the marble through. Then go find some marbles!