Expansion of solids mean that how solids expand. When we heat any solid, then it will be expand means, it has greater mass and weight. So, this process will be understand by observing an simple activity. Expansion of solids only takes place in good conductors of heat.
First of all, you will take one ring and one solid metallic sphere. Check that sphere will pass easily in this ring. Now ring is set in a iron stand and tie one end strictly. Now next step is that you will heat the sphere through any source like spirit lamp, stove etc. The sphere will be heated until the sphere will hot. Now this sphere is put into the that ring. The sphere will not pass in the ring. This means the sphere is expand during heating.
So, when we heat any solid material, it will expand because this is a process of expansion. The wood thermal means "heat". So, only those materials will be take which absorbs heat easily, which are called good conductor of heat. Good heat absorber are those which allow heat to it. So, metallic materials are used to observed these activities.
In other way, expansion of railway lines, expansion of wires and expansion of iron girders are best examples to observed the phenomena of expansion. In railway line the gaps are taken because when light rays are incident on line, then expansion will take place. So, when expansion will start, there is no effect railway line. If gaps are not there, then railway lines bend. And accident will taken.
In this way, everywhere expansion will start whether in your homes or outside. So, thermal expansion of solids have also takes places in other way. This expansion is observed in your houses or in outside your houses. So, i think you will understand the process of heat transfer and how things are expand, when we heat these things. So, thermal expansion only holds in good conductor of heats.