Because engine coolant gets hot. It works through heat exchange - coolant goes through the system, absorbs the heat from the engine, passes through the radiator, then rapidly exchanges the heat to the air which passes through the radiator fins.
Engine is overheating.
It might the thermostat stuck close or the water pump does not work and has to replaced.
It's steam from boiling coolant (or water) coming out of the overflow hose connected to the top of the radiator just below the level of the cap.
Boiling milk contains fats and proteins that create a thicker, stickier consistency when heated, causing it to easily overflow and create a mess. On the other hand, boiling water is a simpler liquid with just water molecules, so it doesn't become as frothy or thick and is less likely to overflow.
The bottle will expand and overflow.
No, it is not safe to put boiling water in a Nalgene bottle as it can cause the plastic to deform or release harmful chemicals.
the water will over fill
It lowers the freezing point and raises the boiling point of water.
First, if you only look after it has disappeared, there is no coolant left to be leaking Second, coolant will often leak only under pressure, when it expands from being hot. This often happens when the car is moving, so cannot be observed, or escapes as steam. These leaks can be from small cracks in hoses, often under the clips; from pinholes in the radiator body; from under the pressure cap if the seal is badly worn. If some coolant is already missing, the remainder will heat up faster, setting up a viscious cylcle. In vehicles with an overflow bottle, overheated water goes to the bottle, to be sucked back later by the reduced pressure as the water cools again. In extreme overheating (boiling) the coolant can overflow the bottle, so less is available for sucking back later. Also if the hose to the bottle is cracked or loose, the reduced pressure will suck in air instead of water from the bottle. This leads to a situation where the radiator is quite low (so susceptible to fast overheating) while the overflow bottle contains water, so the user thinks there is enough water and never refills it. Coolant can also leak into the engine block, but this will not "keep" happening, as quite a small amount will cause the engine to stop functioning.
Make sure the water tank is low enough to take on new water or you might have an overflow problem.
It changes the freezing and boiling point.
The boiling water cools and the freezing water warms up until they are both at the same temperature.