Why do avocados turn brown so quickly — and are they OK to eat at that point?

Person eating avocado toast with poached egg and salmon, close-up view.

The second you cut open an avocado, the clock starts ticking down. Within hours, the fruit goes from an appetizing green to an unappealing brown. But why do avocados turn brown so quickly?

It comes down to chemistry. Avocados have an enzyme called polyphenol oxidase (PPO).