system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why does only half of a wine cork expand when removed from the bottle?

Why does only half of the wine cork expand when removed? Every time I remove the cork from a wine bottle, half of it expands and the other side stays the same size as the bottle opening. I just never understood why that happens. I drink 3 for $10 bottles of wine too so...high class.
gpt
Because the part that doesn’t expand is the top which is dry, the bottom portion absorbs moisture from the wine and expands, or tries to but the bottle holds it tight. In fact this expansion helps the cork seal better. 

Once you pull the cork out the part with the moisture can now expand.