When water is heated, the particles gain kinetic energy and move faster. As the temperature increases, the particles move even faster and collide with each other more frequently. This causes the water to expand and become less dense. At 100°C (212°F) at sea level, the water vapor pressure equals the pressure exerted by the atmosphere, and boiling occurs. At this point, the particles have enough energy to break free from the liquid and escape into the gas phase. The bubbles that form during boiling are filled with water vapor. As the bubbles rise to the surface, they burst and release the water vapor into the air.