Thank you for a great question! As you know, if a balloon is filled with helium and you let go of it, it will float away into the sky because the helium is lighter than air. But lets start at ground level first. When you fill up a balloon and tie it up, the pressure inside of the balloon is equal to the pressure outside the balloon. This means that the air inside the balloon is pushing on the balloon walls just as hard as the air on the outside is pushing on the balloon walls (like a tug-of-war game where both teams are pulling on the rope but it is not moving). As your balloon rises up into the earths atmosphere, the pressure on the outside of the balloon decreases (if youve ever been on an airplane, this is why they have to pressurize the cabin). When the pressure decreases, the air on the inside of the balloon is now pushing harder than the air on the outside which leads the balloon to expand. This keeps happening as the balloon goes higher and higher, and eventually the balloon cant handle so much expanding and it will burst. The pieces would fall back down to the earth.