Nuts and Bolts

At the hardware store, purchase a variety of nuts and bolts.  Put them
in your sand and water table.  Have the children work to find and match the nuts with the bolts and screw them together.  While the children work
at this, reinforce the concept that a nut's outside shapes hexagon. 

If you would like, you can also add peg board pieces or other wood pieces
with holes drilled into them to add to the fun.