Arch Blocks

I really like these arch blocks that I purchased at the Gateway Arch in St. Louis several years ago.  Our faculty traveled there to attend a conference at the New City School on utilizing the theory of Multiple Intelligence.  It is quite a challenge for the children to figure out how to stack these to be able to remove the center block to create an arch.  I love their looks of amazement when the arch they formed stays intact.  I checked the museum store and they still have a version for sale: http://www.jnpa.com/Prod-157-1-504-45/Catenary_Arch_Puzzle.htm