system
Given the following post, respond as if you're commenting on Reddit:
human
Plastic coke bottles (with minimal liquid residue) will, with the cap screwed on tight, develop a partial vacuum (negative pressure) inside over time. Why?   

I've unwittingly been conducting this natural experiment in my dorm room for two years. My girlfriend hypothesizes that whatever chemical or metabolic event taking place in the liquid residue has used the air constituents to form smaller molecules, which results in the negative pressure. 
 
Edit: tomaattisose (and others) has hit on a likely explanation: 
 
> Fluctuations in temperature cause changes in pressure. When the difference in pressure is positive, the air slowly escapes (I bet you don't screw the cap back as tightly when the bottle is already empty.) When the "gauge pressure" is negative, it pulls the cap closing it tighter and thus hindering the flow of air. Also the lack of internal pressure allows the bottle even out the difference by collapsing.
gpt
Fluctuations in temperature cause changes in pressure. When the difference in pressure is positive, the air slowly escapes (I bet you don't screw the cap back as tightly when the bottle is already empty.) 
When the "gauge pressure" is negative, it pulls the cap closing it tighter and thus hindering the flow of air. Also the lack of internal pressure allows the bottle even out the difference by collapsing.