Although air is invisible, it still takes up space and has weight. In this experiment, when the air in the soda can is heated, the air inside the can will rise and some will escape. When the water in the can is heated, it begins to evaporate becoming water vapor, a gas. It fills much of the newly created space left by the escaping air. When the can is placed in the tub of ice water, the water vapor instantly condenses back into liquid water.
What takes the place of the water vapor and steam? Nothing! For a brief second, the inside of the can has much lower air pressure than outside. Air exerts 14.7 pounds of pressure per square inch at sea level (1 kg per square cm). If the air inside the can was at the same air pressure, then the can would remain intact. But with more pressure outside the can than inside, the can is crushed in seconds.
This demonstration works best when students have prior understanding that air and water are made of molecules, and understanding of how water evaporates and condenses.