air molecules. The internal air has to exert force not only to counteract the external air to keep the air pressures "even", but it also has to counteract the natural contraction of the balloon. Therefore, it requires more air pressure (or force) than the air outside the balloon wall. Because of this, when helium balloons are left and they float higher, as atmospheric pressure decreases, the air inside it exerts more pressure than outside it so the balloon pops from tension. In some cases, the helium leaks out from pores and the balloon deflates, falling down. See also Types of balloon Balloon (aeronautics) Gas balloon High-altitude balloon Hot air balloon Observation balloon Research balloon Solar balloon Tethered balloon or moored balloon or captive balloon, a balloon that is restrained by one or more tethers attached to the ground and so cannot float freely Tethered helium balloon Weather balloon Toy balloon Other Aerobot Balloon-carried light effect Flogo Lava balloons List of inflatable manufactured goods Pig Balloon Pneumatic bladder Radiosonde Rockoon Speech balloon Two-balloon experiment Balloon popping References Further reading "Stories Behind Everyday Things"; New York: Reader's Digest, 1980. External links Stratospheric balloons, history and present Historical recompilation project on the use of stratospheric balloons in the scientific research, the military field and the aerospace activity National trade association for the UK balloon industry Balloon and Party Industry alliance for the UK and European Balloon and Party industry National trade association for the Australasian balloon industry Inflatable manufactured goods Party equipment