Why does water boil?
Short answer
Water boils when bubbles of water vapor form inside the liquid, grow, and rise to the surface.
Why do the bubbles grow?
Water can turn into vapor from its surface before it boils. This is evaporation. Water is made of tiny particles called molecules. As water warms, these particles move faster, and more can leave the liquid as vapor.
The pressure exerted by water vapor is called vapor pressure. It rises as water warms. When vapor pressure reaches the pressure around the water, bubbles can grow inside the liquid instead of collapsing.
Why does the boiling temperature change?
Under standard atmospheric pressure (about 1 atmosphere), pure water boils at about 100°C. High in the mountains, lower air pressure lowers the boiling temperature. A pressure cooker does the opposite: higher pressure raises the boiling temperature.