What is thermal expansion from chapter …

Matter expands on heating and contracts on cooling. The expansion of matter on heating is called thermal expansion. There are some applications of thermal expansion in day-to-day life.
1) Blacksmiths use the principle of expansion on heating and contraction on cooling, to fix the iron rim onto the wooden wheel of a bullock cart. The radius of the iron rim is slightly less than that of the wooden wheel. The iron rim is heated to red hot, so that it expands and its radius increases. It is then slipped over the wooden wheel and cooled by pouring water. The iron rim contracts on cooling and gets tightly fixed to the wooden wheel.
2) Gap in Railway Tracks: While laying railway tracks, a small gap is left between adjacent rails. This is because the iron rails expand in summer, and the gap allows space for the expansion.