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?