system
Given the following post, respond as if you're commenting on Reddit:
human
Do balloons filled with helium fly up inside of the iss?
gpt
On earth, helium balloons go up not because of anything specific about helium, but rather just because the balloon is less dense than the air around it, and so that air falls down, the balloon is pushed up. This concept is the same reason why some things will float in water and others will sink. If something is less dense than water, it'll rise up in water, if it's more dense it'll sink.

The important thing, though, is all of this requires gravity. Without gravity, there's nothing to cause things heavier than air to fall down through it, and nothing to cause things lighter than air to rise up through it. In fact, there's not even a terribly good way to define what up or down is. So just as the astronauts will float in the air without falling 'down', a helium balloon will float in the air without rising 'up'.