When matter is heated, its particles gain energy, which is exerted as kinetic energy.
In solids, the particles vibrate harder and faster, creating more space between the particles, causing them to expand. This is most visible in metals. This process is thermal expansion.
In liquids, the particles move around faster, weakening the intermolecular forces of attractions, and are thus held less closely together. The liquid expands. If you want, you can test this out yourself, by measuring and comparing the volume of the same mass of water, before and after heating. A common example is the traditional thermometer – as the bulb of the thermometer heats up, the heat is conducted to the liquid. This causes the liquid to expand, forcing it to rise up the thermometer:
