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We make funny animals out of them. We celebrate birthdays with them. We use them to play games. Balloons are a blast, but they're also super for science. Meteorologists monitor atmospheric conditions by sending up weather balloons. Doctors open blocked veins and arteries by inserting special medical balloons into them. Scientists are even investigating something called superpressure balloons. Sealed against leakage and strong enough to handle the sun's heat, these helium-filled balloons would float in the upper reaches of Earth's atmosphere. They may replace some satellites because they're so much cheaper to launch - just let them go! The material that makes balloons stretch is latex, obtained from rubber trees. Latex is collected as it flows out of cuts in the tree bark. At this stage, it is fairly gooey. People make balloons by dipping a balloon mold into the latex and then heating it for a while to make the latex firmer and more elastic. Latex is a polymer, which means that it has long, chainlike molecules made up of repeating units. When it first comes out of the rubber tree, its molecules are loosely tangled up, so they flow slowly. If you heat the latex, you create chemical cross-links between the molecule strands. Long, tangled polymer molecules that have a few cross-links between them can extend and then regain their original shape. These stretchy materials are called elastomers. The only problem for the serious balloon collector is that the open mesh structure of latex, particularly when it's stretched tightly, lets helium and other gases escape right through the tangled molecules. Mylar, a type of polyester that can be rolled into thin films, is better at trapping helium because its molecules are more closely packed and Mylar does not stretch out like latex. But even a Mylar balloon can't hold all the tiny helium atoms forever, and eventually it loses lift also. Balloons are a great example of how the pressure and the volume of a gas are interconnected. When you blow up a balloon, you exert pressure on the inside walls of the balloon. When that pressure exceeds the outside air pressure plus the pressure exerted by the latex itself, the balloon begins to expand. The pressure inside a balloon is always a little higher than the surrounding air pressure, because the latex is pushing back as the air inside pushes out. When a weather balloon rises in the atmosphere, for example, the outside pressure decreases and the balloon expands. Eventually, the inside pressure causes the balloon to burst.