A hot air balloon is a type of aircraft. It is lifted by heating the air inside the balloon, usually with fire. Hot air weighs less than the same volume of cold air (it is less dense), which means that hot air will rise up or float when there is cold air around it, just like a bubble of air in a pot of water. The greater the difference between the hot and the cold, the greater the difference in density, and the stronger the balloon will pull up. This means that balloons can carry more weight on a cold day, or if the air inside the balloon is very hot. At higher temperature, the air molecules move faster. This causes the molecules to spread out and therefore hot air is less dense than cooler air.