At fairs, carnivals, festivals or anywhere you can buy helium-filled balloons, you're sure to see at least one balloon rising into the sky after someone has accidentally let it go.
What happens to the balloon after it rises out of sight? That depends on how high the balloon goes!
When you inflate a balloon, you're actually putting gases, in this case mostly helium, into an elastic container. As you add more gas, the pressure inside the balloon increases, and in response, the balloon expands...up to a point, that is! If you put in too much helium, and the pressure inside the balloon exceeds the atmospheric pressure on the outside of the balloon, the balloon will burst.
What happens to a rising balloon? Well, if it gets caught by an air current and is carried along level to the earth's surface, it could go a long way, even across the ocean! Of course, no balloon is perfectly sealed, no matter how tightly you tie it, so eventually the helium will leak out, and the balloon will sink.
What if our balloon just keeps rising? Well, eventually, it'll burst. As you go higher into the atmosphere, the air becomes thinner, and the atmospheric pressure drops. The pressure of the helium inside the balloon doesn't fall as quickly as the atmospheric pressure on the outside of the balloon. Before long, there's a lot more pressure on the inside than the outside, and the balloon pops!