When you put water in the radiator and leave cap off while car is running comes back out why does that happen?

When you put water in the radiator and leave the cap off while the car is running, the water comes back out because of the pressure created in the cooling system. As the engine runs, it generates heat, which causes the coolant (in this case, water) to expand. This expansion creates pressure, which is then released through the open radiator cap.

The pressure in the cooling system is necessary to prevent the coolant from boiling. When the pressure is released, the boiling point of the coolant is lowered, which can cause it to boil over. This can lead to overheating and engine damage.

It's important to keep the radiator cap on while the car is running to maintain the pressure in the cooling system and prevent the coolant from boiling. If you need to add coolant, be sure to do so slowly and carefully, and only when the engine is cool.