Not an original idea; taken from a book god-knows-where. It’s a challenge to little kids because the pieces go in only one way, in order. It’s simply three layers of cut out plywood, a bottom and a hinged lid. The three-year old recipient I gave it to, played with it for days before becoming bored, and I’m no expert (unless Grandmother of nine can be an expert) but I think it’s an excellent teaching tool for learning reasoning skills!
Might As Well Dance : http://barbsid.blogspot.com/