collapsing a can

  When you heated the can you caused the water in it to boil.  The vapor from the boiling water pushed air out of the can. When the can was filled with water vapor, you cooled it suddenly by inverting it in water.  Cooling the can caused the water vapor in the can to condense, creating a partial vacuum.  The extremely low pressure of the partial vacuum inside the can made it possible for the pressure of the air outside the can to crush it.

A can is crushed when the pressure outside is greater than the pressure inside. In this experiment, the air was driven out of the can and replaced by water vapor. When the water vapor condensed, the pressure inside the can became much less than the air pressure outside. Then the air outside crushed the can.

When the water vapor inside the can condensed, the can was empty. You may have the water in the pan to fill the can through the hole in the can. However, the water cannot flow into the can fast enough to fill the can before the air outside crushes it.