Substances expand when they get heated up and they contract when they cool down.
Uh? What about the thermometre? How does it measure the substances' temperature, if it is expanding?
But that is exactly because of expansion and contraction that it can tell you the temperature, they work because the liquid inside them rises up the tube when it gets hotter.
But expansion and contraction can also cause problems. Think of a bridge, in Summer, with the heat, it would expand, and that is why we have special joints to stop them bending out of shape.
What is important for you to know is that the particles does NOT grow or shrink, they will always be the same size. The thing that changes is the space between them.
Solids-It's particles vibrate more when is heated and so, take up more room.
Liquids- The particles move more around each other, taking up more space.
Gases- Move more quicklyin all direction when is heated up.