Much of the Netherlands is lower than the sea. Without help, the fields would flood. The Dutch had a brilliant idea: build a windmill where the wind catches its big sails, and use that turning power to drive a big screw or scoop that lifts water up and away.
Each windmill at Kinderdijk is taller than a house. The sails are made of wood and white cloth and can be over 25 metres across - longer than two buses. When the wind picked up, the miller (the person who looks after the mill) would let the sails turn, and the water would start flowing out of the fields.
Windmills weren't only used for pumping water. Some ground wheat into flour for bread. Some sawed huge tree trunks into planks for ships. Some squeezed seeds to make oil. At one point, the Netherlands had around 10,000 working windmills - now there are about 1,200 still standing.
Kinderdijk means 'children's dyke' in Dutch. A dyke is a long wall that holds back water. The 19 windmills there are nearly 300 years old and a UNESCO World Heritage Site - which means they are looked after for the whole world, not just the Netherlands.