Lighter-than-air flight was the first method used to take to the skies. Air that is less dense (or "lighter") rises. Heating the air inside of an envelope (or balloon) makes the heated air less dense, thereby causing it to rise. Another method is to fill the envelope with low-density (or "light") gas such as hydrogen, which is very flammable, or helium.
There are two basic types of lighter-than-air vehicles -- balloons and dirigibles. Balloons are either tethered or drift with the wind, and the pilot can only control altitude. Dirigibles, often called airships, are powered, light-than-air vehicles that can be steered.
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