Balloons are common decorations, found everywhere from birthday parties to car lots. In order to keep these balloons floating, they're filled with gas that's lighter than air. The most popular of these gases is helium, usually kept in a compressed tank. However, an odd phenomenon occurs regarding helium filled balloons when it gets cold... for some reason, they shrink.
Balloons
A balloon is defined as a flexible bag that's designed to be filled with a gas. Most balloons are made of rubber, and are filled with helium. Balloons which are filled with helium will rise, whereas those which are just filled with air from a person's lungs will not. The reason for that has to do with the gas used, and its weight, or density.
Helium
Helium is a colorless, odorless, naturally occurring gas. It's most notable use is as a balloon filler, followed by a use in refrigerants and laser media. The reason that helium is so popular in balloons is that it causes them to float. The reason for this is because helium is a "lighter than air" gas. But it would be more accurate to say that helium is less dense than air, which makes it buoyant.
Density
What makes helium filled balloons float is the density of the helium inside of them. Air has a certain density, and anything with a greater density than that air will sink. However, anything that has a lower density than air, such as helium or heated air, will rise and float. It's helpful to think of density in terms of water. Those objects which are denser than water will sink, and those which are not will float. While simplified, this applies to air as well.
Temperature
Helium is greatly affected by temperature. When helium is heated, it causes the molecules to move faster and with more energy, and the gas expands. When helium is cooled though, the effect is entirely the opposite. The molecules become less energetic, and the gas shrinks. If helium is filling a balloon, and the gas shrinks, then it won't stretch the balloon out as much, and the balloon too will shrink.
Other Gases
The expansion and shrinking of gas due to heat isn't unique to helium. A hot air balloon for example will float because the air inside of it is heated till it expands. Because of the heat, the air in the balloon is of a lower density than the colder air surrounding it. If that air cools though, it will shrink, and cease floating.
