How do solids expand? This question is often asked because it defies rational surface thought that's based on the assumption that solids are solid because they are hard and they don't move. After all, it is difficult to look at a block of iron and imagine that it would move at all, but it does in unnoticeable ways, by both expanding and contracting.
The basic explanation for the expansion of solids has nothing to do with the quantity of the solid in question, but the molecules it is made up of. These molecules, which are composed of various atoms, make up all forms of matter and by their separation from one another, determine the phase of that substance. The three phases are solid, liquid, and gas, and are achieved by differing levels of heat.
With lighter elements (such as the common elements in air) they will primarily exist in the gas state, while heavier earth atoms may never reach a gas state, due to the amount of heat needed to reach vaporization. The various changes of state move from solids to liquids to gases based on additional heat, thus increasing the ability for molecules to move around or flow. The opposite state change is done by removing heat from a system, and decreasing the amount of flow. Most of the solids that people are used to interacting with have very high freezing temperatures, in which they take a non-flowing shape, but are not completely unmoving.
In the expansion of solids, enough heat is added to a solid mass that entices the molecules to move apart. Although these molecules do not move enough in order to induce a phase change to a liquid state, they cause the substance in question to increase in size by swelling. This swelling is expansion. When enough heat is removed from the same substance, it will contract and become more rigid, and possibly brittle.
Examples of expanding and contracting substances with the addition and removal of heat is easy to see in any house. Creaky floorboards or the groans and noises during the night can be attributed to contraction and separation of the boards to each other. During the day this is not the case because they expand much more. Glass casserole dishes that have been baked within an oven at high temperatures expands to a point that if it cools too rapidly (by placing it on a countertop) it will condense faster in one point than the next, and break as a result. Metal utensils placed against frozen carbon dioxide (dry ice) will emit a high pitch squeal as the metal rapidly contracts, only to expand again when removed from the cold source. In places outside the home, other examples of the expanding and contracting metals can be seen in the joints on either end of a bridge, with are demonstrated by the metal teeth interconnected. When it expands in the summer it has room to expand outward, and in winter it will be able to shrink accordingly, so as to not break the concrete.