system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What happens that makes beer taste terrible after warming up and then re-chilling? What makes beer 'skunky'?
gpt
Skunking occurs when beer is exposed to certain wavelengths of light. This is why beer is typically bottled in brown bottles. Clear and green bottles protect the beer less, which is why popular brands in those color bottles often have the distinctive flavor. 

The chemical responsible for the distinctive flavor is called MBT, and is produced when the energy in light causes the riboflavin (produced by yeast during fermentation) in beer to chemically react with the alpha acids from the hops.