Thermal Expansion
Thermal Expansion is when things get bigger because they are heated up.
Think of it this way
A small metal lid sits tight on a glass jar and will not turn. You hold the lid under hot running water for a few seconds. The metal heats up and its tiny parts spread apart, making the lid just a bit wider. Now the lid turns with ease — it grew from the heat and no longer grips the jar as tight.
Explaining thermal expansion by grade level
Put a balloon over a bottle and set the bottle in warm water. The balloon starts to puff up. The air inside the bottle gets warm. Warm air takes up more room, so it pushes into the balloon.