Matter (all the ÒstuffÓ in the universe, as opposed to just empty space) comes in three main forms: gases, liquids, and solids. All matter is made up of very tiny particles called atoms. The main difference between these three states of matter is how fast their atoms move, and how much space is between them. In gases, the atoms move very quickly, and there are wide spaces between them. In liquids, the atoms donÕt move as quickly and are closer together. The atoms of solids are slowest and most crowded together.
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One way to change matter from one state to another is to alter its temperature. When you heat something, you speed up its atoms, which causes them to bounce off each other more quickly and creates more space between them. When you cool something, you slow down its atoms, which makes them bounce off each other more slowly and crowd closer together. This is why water becomes a solid (ice) when it is stored at its freezing temperature (0¡ C or 32¡F) and why it becomes a gas (steam or water vapour) when it is stored at its boiling temperature (100¡ C or 212¡F).
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Oxygen and hydrogen both have freezing and boiling temperatures, too. They are much lower than those of water. Oxygen freezes and becomes solid at -219¡ C (-362¡F). It boils at -180¡ C (-292¡F). The freezing point of hydrogen is -259¡ C (-434¡F). It boils at -253¡ C (-423¡F). These temperatures are much lower than any that occur naturally on Earth.
