Take an empty aluminum soda can and add a few drops of water to the inside of the can. Cover the opening with a piece of scotch tape, then puncture a small (pin-size) hole in the tape to allow a small amount of airflow. Now fill a small bowl with about an inch of cold water. Heat the soda can until you start to see a little steam coming through the hole in the tape. Carefully (be sure to use protective gloves or tongs) invert the hot can into the bowl of water so that the taped end is submerged. You may observe something fairly interesting.

Before heating, the can is filled with water and air. By boiling the water, the water changes states from a liquid to a gas. This gas is called water vapor. The water vapor pushes the air that was originally inside the can out into the atmosphere. When the can is turned upside down and placed in the water, the mouth of the can forms an airtight seal against the surface of the water in the bowl. And then just after that, all of the water vapor that pushed the air out of the can and filled up the inside of the can turns into only a drop or two of liquid, which takes up much less space. This small amount of condensed water cannot exert much pressure on the inside walls of the can, and none of the outside air can get back into the can. The result is the pressure of the air pushing from the outside of the can is great enough to crush it.

To make the internal pressure of the can balance with the external pressure on the can, the can implodes.

The outside air pressure pushing downward on the surface of the water is greater than the force inside the can and the water gets pushed up into the can.