Charleon

Meet Charleon, a balloon who grows when the gas inside her warms up.

Charles's law says that for a fixed amount of gas at constant pressure, the volume is proportional to the absolute temperature. Double the temperature in kelvin and the volume doubles. Jacques Charles studied this in the 1780s, and Joseph Louis Gay-Lussac published careful measurements in 1802.

The temperatures must be in kelvin, not degrees Celsius. A 3.0 L balloon at 300 K warmed to 360 K grows to 3.0 × 360 / 300 = 3.6 L. Put a balloon from a 25 °C room (298 K) into a 0 °C freezer (273 K) and its volume shrinks to about 273 / 298 ≈ 0.92 of what it was.

The reason is molecular. Hotter gas molecules move faster and hit the walls harder, so to keep the pressure the same, the gas spreads out until the hits are less frequent.

Hot-air balloons use this. Heating the air inside makes it expand, so some of it leaves through the opening. The air left inside is less dense than the cooler air outside, and the balloon floats.