Definition
Thermal expansion is when an object's length or volume grows a little as its temperature rises. The tiny particles inside it jiggle harder and push slightly farther apart. When the temperature drops, the object shrinks back a little.
Spread Your Arms and the Line Gets Longer
Picture your gym class lined up in a row. On a freezing day, everyone hunches their shoulders and stands close together. Then they warm up and get excited, and they start swinging their arms and taking more space. The number of people is the same, but the whole line is noticeably longer.
The same thing happens inside an object. An object is a clump of tiny particles, each one held to its neighbors like people holding hands. A higher temperature means those particles jiggle harder in place.
The harder they jiggle, the more room they need to avoid bumping into each other. The particles themselves don't puff up. Instead, the gaps between them get wider. Those tiny gaps add up across the whole object, and you see it as extra length and volume.
Why Railroads and Bridges Have Gaps
The more people in the line, the more the line grows when everyone spreads their arms. One person's extra arm space is tiny, but in a long line those small bits add up. Objects work the same way: the longer they are, the more they expand.
A 1-meter steel rod heated by 100 degrees grows by just over 1 millimeter. You'd never notice that by hand. But at the same rate, a 1-kilometer railroad track would grow by more than a meter.
A track that is squeezed tight with nowhere to go can buckle sideways and pop upward. That's why railroads and bridges are built with joints that leave room to expand in advance. The gap narrows in summer and widens in winter, and that small space prevents accidents. On high-speed rail, long welded rails are clamped down firmly so the middle can't stretch, and expansion joints are placed only at the ends.
The same idea is behind the sliding supports placed under long pipes and bridge decks.
A Little More Precisely
Back to the railroad for a moment. When an object heats up, the whole thing scales up by the same proportion, like zooming in on a photo. That means a hole in the middle grows too. This is why a stuck metal lid on a glass jar loosens when you dip it in hot water. The lid's inner edge gets bigger, and since metal expands more than glass, a little gap opens up.
How much something expands depends on the material. Heat metal and glass by the same amount, and the metal grows a lot more. If you bond two different metals into one thin strip, one side expands more and the strip bends by itself. This trick is used in thermometers and in switches that cut off electricity.
Water bends this rule a bit. Over most temperatures it expands as it warms. But between 0°C and 4°C, it does the opposite and expands as it gets colder. That's why water is densest at 4°C. Ice taking up more space is a separate story: when water freezes, its particles line up in a looser pattern. That's why a bottle full of water can burst in the freezer.
So the rule that things expand when heated works well for most solids and liquids. But to know how much, you have to look at the material.
Common misconceptions
If you heat a metal plate with a hole in the middle, the hole gets smaller.
The hole grows too. The whole object scales up by the same proportion, like zooming in on a photo, so the empty space gets bigger by that same proportion.
Thermal expansion only happens in hard objects like metal.
It happens in liquids and gases too. In fact, gases change volume with temperature the most.
Where you meet it
If you dip a stuck metal jar lid in hot water for a moment, the lid expands more than the glass and turns easily.
Power lines look saggier in midsummer because the heat has made them longer.
In one sentence
When temperature rises, particles jiggle harder and spread farther apart, and those tiny gaps add up to more length and volume.