system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why do teabags swell up and get full of air when I’m pouring hot water onto them?
gpt
Air easily passes through dry fabric (or paper), but once it's wet, the water's surface tension prevents air from passing through it. So, a wet tea bag is (somewhat) airtight. If the tea bag gets wet while it has air in it, the air can't get out. And the air expands as it gets heated by the hot water.

Whereas if you lower the tea bag gently into water, the air escapes from the top of the bag which remains dry until the entire bag is submerged in water.