Why do some beers smell skunky?

Answer: Light reacts with hop compounds

Light reacts with hop compounds ✓ — Correct! 'Skunked' beer is light-damaged, not old. UV and blue light react with hop compounds (helped by riboflavin, a vitamin in beer) to form MBT, a molecule almost identical to skunk spray. Brown glass blocks that light; clear and green bottles don't, and cans or kegs are safest.

The beer has gone old and stale — Wrong. Skunking is about light, not age. A fresh beer left in sunlight can skunk in minutes, while one kept in the dark stays fine far longer.

Bad water was used to brew it — Wrong. It's not the water. The skunky smell is a light-triggered reaction with hop compounds, not a brewing-water problem.