Because it's leaking somewhere. Other than lines in the cooling system, head and intake manifold gaskets are a possibility.
check hoses coming from coolant tank by sqeezing may be worn
you are low on coolant.
If you mean engine coolant is getting into the car, the heater core is leaking coolant causing the coolant level to drop and that causes the engine to over heat.
If your talking about coolant leaking on the passenger floor then it means your heater core is leaking and needs to be replaced.
Assuming there is coolant in the engine the most likely cause is a build up of sediment behind the coolant drain or in the coolant drain tap itself.
Probably a coolant leak.
It could be symptomatic of a larger problem. The coolant may not be circulating properly, or the indicator panel or car's computer could be developing trouble.
If you can't see where its coming from, it may be a cracked head, or a bas headgasket. Also, when you add water to the coolant tank, it doesn't immediately go into the radiator. After the car sits a while, the radiator starts to cool, creating a vacuum. Water/coolant is then pulled into the radiator from the coolant tank.
No , this coolant is coming from your heater core and you are losing engine coolant ; you need to have this repaired because the engine will overheat and suffer damage that will be more expensive than the repair of the heater core .
Could be overheated coolant or just leaked oil burning off your exhaust headers
The bigger your car, the more coolant you need to put in. The bigger your engine and car, the more coolant you need
No.