Why Do Cut Avocados Turn Brown?
Avocados contain an enzyme called polyphenol oxidase, which when in contact with oxygen in the air, causes avocados to turn brown. Really the best way to keep an avocado from turning brown is to reduce its contact with air as much as possible. The methods you're most familiar with all attempt to do this, but many fail.
The Pit Method
Most food specialists will claim that leaving the pit in an avocado will keep it from turning brown, or placing the pit in a bowl of guacamole will keep the dip from turning brown. This is only partially true. Yes, if the pit was not detached from one half of the cut avocado, the flesh that stays in contact with the pit will not turn brown. And anyway it takes about 6 hours for a cut avocado to start turning brown when it sits at room temperature. That is because it hasn't come into contact with oxygen. Just think, when you make guacamole and place a pit in the bowl, you also tend to cover the dip with plastic wrap, right? It's really the plastic wrap that keeps the guacamole from turning brown, not the pit! So this method is false.
The Citrus Method
Acid does help keep an avocado from browning, but really it only slows down the process. If you're only eating half an avocado, say for breakfast or lunch, keep the pit attached to the second half and rub the cut side with a little lemon or lime juice. Place it in a resealable plastic bag, removing as much air as possible, and refrigerate. It should be fine for up to 2 days, but you might need to scrape off some discoloration. For diced or sliced avocado, Alton Brown recommends tossing with 1/2 teaspoon lemon juice per 1 avocado. Cover it with plastic wrap so it touches the surface of the avocado and store in the fridge. This method works in combination with sealing out air.
The Water Method
Would you believe that cold water helps keep an avocado from turning brown? Chefs and cooks use this method because they need to prepare avocados in advance of restaurant service. Whether you dice, slice or just spoon out your avocado halves, you'll want to immediately immerse them in ice cold water (they'll stay fresh for up to 4 hours). The cold water keeps the avocado away from oxygen. Also, rinsing the avocado under cold water helps delay browning. This method works because it keeps out all oxygen.
So whichever method you use, it's all about keeping the avocado away from oxygen. Plastic wrap was never so handy!