Where do you put water in a car engine?

Straight water should not go into an engine at all, but people can use a mixture of automotive engine coolant and water which can be poured into the car's radiator using the radiator fill cap.

The problem with using regular water in a radiator is its much lower boiling point. Engine coolant is much denser and thus much more effective at dispersing heat from the engine without building up pressure inside the radiator. Too much water in the coolant mixture can also have detrimental effects as well, as boiling water expands in a vapor and can burst radiator seals. Most engine coolants have directions for appropriate mixtures and specific water to coolant ratios.