Radiator spews fluid out when changing water?

The cooling system on you car's engine is pressurized to reduce the evaporation temperature of the fluid, to make the system more efficient. If you open the radiator cap when the engine is hot, the fluid sprays out as the pressure equalizes. This can burn the s##t out of you, seriously. It's dangerous and wasteful of the coolant fluid. WAIT, an hour or so, to let the engine and readiator cool off, before taking off the cap. Then you can add water/coolant mixture. DO NOT PUT ONLY WATER into the system! Use a mixture of coolant and water, or straight coolant from the bottle.