A hammerstone is a really old tool that our ancestors used a long time ago. It's made of a big, heavy rock or stone that they would use to hit other rocks or stones to shape them into different things.
It's a bit like when you use a hammer to hit a nail into a piece of wood, but instead of nails and wood, they used the hammerstone to shape different tools and weapons out of rocks.
People would hold the hammerstone in one hand and use it to hit the rock they wanted to shape in the other hand. And because the hammerstone is really heavy, it would create a lot of force and help break off pieces of the rock.
It might sound simple, but it was a really important tool for our ancestors because it helped them create all sorts of things they needed for survival, like knives, arrowheads, and even hunting tools.
Nowadays, we have different tools to do these things, like hammers and saws, but the hammerstone was one of the first tools ever used by humans a long, long time ago.