What happens when water boils
Everything is made of tiny particles, each ATOM or MOLECULE constantly jiggling. In a SOLID they are packed tightly and only vibrate. In a LIQUID they slide past each other, so it can FLOW and take the shape of its container. In a GAS they fly about freely and spread out to fill any space. Adding HEAT makes them move faster, and that is what drives each CHANGE of state.
Heat a pan of water and the molecules move faster and faster. At 100°C, at sea level, some have enough energy to break away from the liquid altogether and form bubbles of water VAPOUR inside it: that is BOILING. The big bubbles are not air; they are water turned to gas. The clouds rising from a kettle are not really STEAM either. Steam itself is invisible; the white cloud is tiny droplets that CONDENSE as the steam hits cooler air.
Why tea is weak on Everest
The temperature at which water boils depends on the air pressing down on it. High on a mountain, where the air is thinner, water boils at a lower temperature: on the top of Everest it boils at about 70°C, too cool to make a proper cup of tea or cook pasta quickly. A pressure cooker does the opposite, trapping the steam so the pressure rises and water boils at around 120°C, which cooks food much faster.
Water can EVAPORATE at any temperature, though, not just when it boils. Puddles dry up on a cool day because the fastest molecules at the surface keep escaping.
Ice floats, and that matters
Almost every substance shrinks as it turns from a liquid to a solid, so the solid sinks. Water is unusual. When it goes through FREEZING, its molecules lock into an open pattern that takes up about nine per cent more space, so ICE is lighter than water and floats. That is why pipes can burst in a hard frost. It also saves life in ponds and lakes, which freeze from the top down, leaving a layer of liquid water beneath the FROZEN surface where fish survive the winter.
Some solids skip the liquid stage altogether. Solid carbon dioxide, called dry ice, turns straight into gas, making the fog used on stages. This is called sublimation, and it is why snow can SUBLIME away on a cold, dry, sunny day without ever MELTING.
The fourth state, and a myth about glass
There is a fourth state of MATTER, called PLASMA. Heat a gas enough and the atoms lose some of their electrons, making a glowing soup of charged particles. The Sun and the stars are plasma, so is the glow inside a neon sign, and it is the most common form of visible matter in the universe. Our chemistry page covers what those atoms are made of.
People sometimes say that old church windows are thicker at the bottom because glass is a slow-flowing liquid. It is a myth: medieval glassmakers could not make panes of even thickness, and glaziers usually fitted the thick edge at the bottom. Glass at room temperature would take far longer than the age of the universe to sag noticeably. The VOLUME of a gas, a liquid or a solid, and the particles inside, are the same whatever the state.