For this flannel board I made the tool boxes out of a front and back piece so that the hammer can be hidden inside. I thought it would be more fun to peek inside to see if the hammer is there rather than just looking behind it. This flannel board has been done by many people and I borrowed my wording from Roving Fiddle Head.

I Lost My Hammer
Place all toolboxes on the flannelboard. Hide the hammer under one of them.
“I built so many toolboxes that I cannot remember in which one I put my favorite hammer. Can you help me find my missing hammer? In what toolbox should we look first?”
After a color is chosen, say the following phrase before removing that color toolbox.
I need my hammer lots and lots.
Is it hiding in the (say colour) toolbox?
Repeat until the hammer is found.