An old mill is a big machine in a building. It grinds grains like wheat or corn into flour. Picture a tall, strong building made of bricks or wood, with a big water wheel on one side. The wheel turns because of water from a nearby river.
This makes the mill work inside. Inside, there are many gears and stones that crush the grains to make flour. Sometimes you can hear the noisy machinery. Around the mill, you might see sacks of grains waiting to be ground and workers busy working. In the past, old mills were really important for making food. Nowadays, some old mills are still here, but many have become museums or places for tourists to visit.