What do you make with a hammer?

Jacob is happily playing in his room while I am cooking supper. I periodically check in on him to make sure he's not grinding playdoh into the carpet or has hurt himself. I walk into his room about 5 minutes ago and he's hitting his Connect 4 box with a toy hammer. He looks up and says, "I'm hammerin'!" I say, "oh yeah?" He says, "yeah, you make a house with a hammer." He's so smart!