Testing the functional prototype helped us realize that kids figured out very quickly that they could sort by size rather than item. It’s also problematic that the small balls can fit in the larger holes, even though they haven’t been sorted correctly.

We considered a lot of ways to sort using different methods—shape, color, weight, magnet—but eventually settled on a plan to use barely different sizes to sort, and to sort items twice so that smaller items end up in a “try again” pile when they are put in large holes.