Air Pressure: Collapse a Milk Jug

Heat some water to near boiling. Pour the hot water into the milk jug and swirl it around to heat the entire container. Pour the water out and quickly cap the milk jug. Within a few minutes, as the air inside the jug cools, the jug will collapse.

The hot water heats the air inside the jug, causing the air molecules to move faster and some of them move out of the jug. By capping the jug, you don't allow any air molecules to re-enter the jug and have created an area of low pressure. The pressure outside of the jug is greater, causing it to collapse.