If I let a balloon go, how high can it get before it pops, and why?
At sea level there is about one ton of pressure exerted on you by the air at all times.
This pressure comes from the 100 kilometres (60 miles) of air above weighing down the air below. As helium balloons rise, the atmospheric pressure drops as there is less air above, causing the balloon to expand.
This expansion will continue until either the gas inside the balloon is of equal density to the air outside the balloon, or it pops due to the latex not being able to stretch any more (the latter being the most likely scenario).