Avocado pits keep guacamole from browning
Many people seem to have their favorite methods to keep avocados from turning brown. One popular method is to leave the pit in the bowl of guacamole. The scientific reasoning is that avocados have an enzyme called polyphenol oxidase (PPO) which causes the fruit to brown when it comes in contact with oxygen. The pit protects the guacamole only because it shields that little portion of the dip it touches from the oxygen in the air.
Other anti-browning suggestions include sprinkling with lemon juice, covering it in a water bath, or wrapping it tightly in aluminum foil. The best method may simply be to eat your guacamole faster.