If too much coolant is put into the cooling system of a car, the extra fluid flows out of the overflow pipe. Coolant and antifreeze, as well as many other car fluids, are highly toxic and can be harmful to children and pets.
If there is too much liquid, it is flushed through the overflow pipe. If it is low, additional coolant should be added.
Due to the heat and pressure of the radiator, it is best to work on the system when the car is cool or use a cloth to protect skin from contacting the cap. Add equal amounts of coolant and water until the level is near the "MAX" line on the reservoir.