After putting ice-cold water in a thermos, the cap is strongly held on by what feels like suction. I assume the cold water condenses the air, thus lowers the pressure inside, but can someone explain why this causes suction?
When the thermos is closed, the air at the top of the thermos will be warm relative to the ice-cold water below it. As time proceeds, the two materials that are in contact with each other at the surface of the water will exchange heat due to their different temperatures. This will cool the air.
When gasses are cooled they move more slowly and they don't collide with surfaces as fast. Pressure comes about from the number of molecules striking a surface per second AND the speed at which they strike it. The molecules striking a surface per second will remain approximately the same, but with lower speed, the pressure produced will be reduced.
Meanwhile on the other side of the closed cap, the molecules will have the same speed they have always had, and they will exert normal atmospheric pressure on it. Hence, the DP that is observed and the apparent "suction" that you describe.