Why do steam and hot water spray from my thermos flask? I put nearly boiling water in a Thermos flask (trade name for a vaccuum flask for home use). Later when I unscrew the stopper, steam and droplets of hot water are forcefully sprayed out, proving that the pressure inside the flask is higher than ambient pressure.
But how can that possibly be? When I poured the water in, all the air above the water but inside the flask was at atmospheric pressure (if the heat of the boiling water I was pouring in had raised any of it above ambient pressure, it would instantly have escaped through the opening at the top). Heat energy escapes from the flask while the stopper is on, as proved by the fact that the stopper feels very warm while it is screwed down. 
So how on earth can the pressure inside a cooling flask rise from atmospheric, when I screwed the stopper down, to above atmospheric when I unscrewed it?
 A: The air in the flask was cooler than the water when you closed the lid.
A: Yea, air inside and flask was cold, but what about equilibrium vapor pressure? If you have air humidity below 100% in that too, and even if u put 25°C water inside it, sooner or later it will build up pressure due to that. If you put in it boiling water it will be sooner. Also i would say that Boiling water is like Carbonated water, but reaching 100% humidity below boiling is very slow process (like 4x slower each 10°C)
Source: Equilibrium vapor pressure, Pressure of water 
What i wanted to say: If you put no air, or air with 100% humidity and same temperature as water - there is no way to create more pressure.
A: When you closed the lid you left say 25 C air alone with boiling water. After a while they will reach an state of thermal equilibrium, where some energy transfer to the air from the boiling water, therefore as a result of increase in temperature, this air will get expanded and needs more space but there is no place to expand in the closed flask so the pressure of air will increase and when you open it, it can get free.
