They tell us how to dress in the morning and when to stop roasting a turkey. When we catch a cold, they tell us how sick we are. Thermometers are an everyday part of our lives, something we usually take for granted. Temperature affects human beings, making us shiver when we’re cold or sweat when we’re hot, but it also affects every other physical substance in the universe. The most common effect has to do with volume, or the amount of space something fills up. Generally speaking, increasing temperature will cause most substances to expand, while decreasing temperature will cause them to contract. The reason for this is that everything is made of tiny bits of matter called atoms. Even if something isn’t moving, its atoms are never completely still. The atoms continually wiggle around and bump into each other. As a substance gets hotter, its atoms start jostling about more vigorously.