CLEVELAND -- Holy guacamole! As U.S. sales of avocados continue their upward rise, the trendy green fruit has a downside.

Or more like a brown side.

It doesn't take long for oxidation to turn that freshly cut, pretty, green color into an unappetizing brown.

You can squeeze limes or lemon juice on them, wrap them in plastic, store them in cold water, or leave in the pit - which many Mexican grandmas swear by.

But sorry, Abuela!

We tested all four methods to see which worked best

After 24 hours, all of our avocados showed significant browning. The one stored in cold water fared the worst and turned into a mushy, brown mess.

However we tested another method, and found that two days later -- our avocado was still pretty and green.

The secret? Onions.

Chop a fresh onion (Don't use leftovers!) and store a few large chunks with the cut avocado inside an airtight container. The freshly cut onion releases syn-Propanethial S-oxide, a sulfur gas that slows down the oxidation process.

Use a milder red onion if you're worried about an onion taste.