Shape Mistake: noun. The mistake a child makes when confusing one object with another because of the similar shapes.
Monkey Son #1 and I were making an Advent Calender out of construction paper. We made a Christmas tree and some little paper ornaments with numbers on them with the idea that we put up one ornament per day until Christmas. What confused me, though, was that when Monkey Son #2 saw the ornaments on the table, he went up to them and seemed to want one, saying "Fank oo (Thank you)."
I didn't know why he wanted one, but I gave him one that had not been colored. Again he said, "Fank oo," and toddled off. When he was a few steps away, he stopped and tried to take a bite. He pulled back out of his mouth immediately when he realized it was not a ginger snap.