What happens when a gas is cooled and compressed? 

Take marbles up to the $50 \mathrm{ml}$ mark in a graduated jar. Add $50 \mathrm{ml}$ of water to the jar. The water level will not reach the $100 \mathrm{ml}$ mark. Explain why. Which concept is illustrated by this experiment? 

If you hold a piece of ice in your hand, it soon melts. At the same time, your hand becomes cold. In terms of energy transfer, explain why your hand becomes cold at the same time as the ice melts.