Helium is less dense than air and therefore rises. When it rises to high altitudes, the pressure inside the balloon is greater than the atmospheric pressure which causes the balloon to pop.
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Because helium is lighter than air, a helium balloon is able to rise through the air towards higher altitudes. However, at higher altitudes, the atmospheric pressure is significantly weaker. Therefore, when the pressure of the helium gas exceeds the atmospheric pressure, the balloon pops.