system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why can you stick different parts of your body(head, hand, finger) in small gaps really easily but you can't take them back out?
gpt
One of the reasons (besides the mentioned swelling) is that when you push, say, an arm through a gap, you are stretching skin and flesh (think pulling up a sock), which makes the limb thinner.

When you are pulling it out, skin and flesh crumples in the gap and makes the limb thicker.

To see it in an easier way: Wear a loose sock in your hand, and push it through a gap (for example between two couch pillows). Its stretches going it, but crumples when pulling out.