Do a small experiment, pour a small amount of hot water into a plastic beverage bottle, shake it, pour out the hot water, and tighten the bottle cap immediately. After a while, you will understand.

What you see is that the plastic bottle is flat. The reason for this phenomenon is that it expands with heat and contracts with cold. Because the motion of atoms in the object changes with the temperature, the vibration amplitude of gas particles in the bottle increases after pouring hot water, and it expands when heated. After the bottle cap is tightened, when the temperature returns to normal temperature, the vibration amplitude and volume of the particles will decrease.

In this way, the air pressure in the bottle becomes smaller and flattens until the air pressure inside and outside the bottle reaches a balance. Thermal expansion and cold contraction refer to the characteristics that an object will expand when heated and contract when cold. Because the motion of particles (atoms) in an object changes with temperature, when the temperature rises, the vibration amplitude of particles increases, which makes the object expand; However, when the temperature drops, the vibration amplitude of particles will decrease, thus causing the object to contract.

Extended data:

Thermal expansion and cold contraction are the characteristics of general objects, but water (below 4℃), antimony, bismuth, gallium, bronze and other substances contract when heated within a certain temperature range, and expand when cooled, which is just the opposite of the characteristics of general objects. Therefore, when water freezes, ice first appears on the water surface.

When water freezes, ice first appears on the water surface. Because the rail expands when heated and contracts when cooled, it is necessary to leave a certain gap when connecting the rail (to prevent the rail from being pushed and deformed when heated and expanded when the temperature rises), and then connect the rail with fishplates and screws.

Baidu Encyclopedia _ Thermal expansion and cold contraction?