Why do apples turn brown when cut?

Answer: Oxygen triggers a chemical change in the fruit

Water loss makes the cut surface dry out — Not quite. Drying out can make the surface look dull, but the browning itself is a chemical reaction, not just water loss.

Oxygen triggers a chemical change in the fruit ✓ — Exactly! When you cut an apple, oxygen from the air reacts with compounds inside the fruit. An enzyme speeds up this reaction, and the resulting brown pigments are what you see.

Sunlight damages the exposed apple flesh — No, light isn't the culprit. The browning happens even in the dark, as long as oxygen is present.