In our round of unpacking, I finally found Lucy's old shape sorter. It's wooden, and sturdy, and the circles and squares and triangles make a satisfying thunk when they drop in the box. The top with the shapes cut out of it is yellow.
We handed it to Julia, gave her a circle, and waited to see what happened.
She took the circle, banged it against the trianglular hole a few times, then lifted up the yellow top and put the circle inside. Then she closed the lid and crawled away.
That's what I call thinking outside the box.