Chemical Concept Demonstrated: Atmospheric pressure
After the can is stoppered, it collapses.
The water inside the can is boiled. The water takes up more space as a gas than as a liquid, and this forces much of the air out of the can. When the heat is removed, the water returns to its liquid state and air, under normal circumstances, would be able to fill the space in the can again.
However, a stopper was added, preventing this from happening. Instead, the water returns to its liquid state, but the stopper prevents air from filling up the leftover space. This leaves whatever little air happened to be in the can after stoppering the task of filling the entire leftover space above the water. Because there is less gas per unit area inside the can than outside the can, the pressure inside the can is less than the pressure outside the can.
The greater pressure outside of the can pushes in on the can, and the lesser pressure inside of the can is unable to push back with equal force. The can, as a result, collapses.