[Moderator's note: spin off from another thread on a different topic.] "why shower curtains blow inwards..." This phenomena is likely caused by warm air [due to hot water spray] rising and pulling in cooler air from the room. It's the same idea as cooler denser air being closer to a floor, warmer air closer to your ceiling. I would not ascribe this phenomena to 'gravity' except to note that's why the curtain is hanging vertically in the first place. Without gravity, it would be floating around as you may have seen in pictures inside a space station where pencils float. Why things float that way in a space station IS an interesting phenomena.