 Why is gap left between rails on railway tracks? Why do electrical overhead wires hang loosely instead of being stretched tightly? Why one end of the iron girders is placed on rollers in construction of bridges? Why are the glassware used in kitchen made up of Pyrex glass? The answer is only one, thermal expansion. Let's understand thermal expansion with the help of a simple experiment. If I take the iron bar and try to fix it into the gauge, you can see the iron bar exactly fixed into the gauge. But if I start heating it up and what will happen after heating? The size of the iron bar will be increased. So after heating, if I try to take the same iron bar into the same gauge, you can see after heating the bar does not fit into the gauge. Let's see what happens inside the molecule. If we start heating up the iron bar, the intermolecular distance of the iron bar is going to be increased because they gain thermal energy. Because of thermal energy, the bar got extended. What is the result of this expansion? The length of the bar and even the volume of the bar is going to be increased and the bar will not be maintained its same size. In case of liquid, the liquid molecules if you provide heat to it, it starts vigorously movement and they start moving like this and striking in such a manner. What happens after some time? It's gain more energy and it starts moving more and more faster. Similarly, if you compare the solid and liquid expansion, you can see in solid the particles are vibrating in a fixed position and in liquid there is a vigorous movement of the particles. Let's understand the gas. If you provide heat to the gas, the bubbles are going from the delivery tube to the other end as you can see here and bubbles are made in the beaker. If you turn off the burner, you can see the same gas is coming back from the beaker. You can see the bubbles here from the beaker back to the container. Let's see how the molecules behave inside that. The molecules will be randomly moved in the gas particles. They gain energy. They are striking each other like this. Let's revise the expansion of solid liquid and gases. If I increase temperature from cold to hot, what will happen to the molecules? They vibrate it more. If I come back to cold, they are closer, hotter, they are far away with each other. They vibrate it more and more by gaining thermal energy. In case of liquid also, there is a vigorous movement. If I increase the temperature, the movement is increased. If I decrease the temperature, the movement is decreased and gas, the maximum expansion is going on. There is a random movement. They got high speed in case of a temperature. They are striking on the ball of container. And if I make it cold, they slightly come back to each other.