The world around us is jam-packed with information—if you know where to look. Take that little sticker on your Granny Smith, for example: That four- or five-digit number is a code that can tell you more about your produce than you ever wanted to know.

Price Look-Up (PLU) numbers have been around since 1990, and they serve two functions: First, they allow your cashier, or you (if you’re the self-checkout type), to, well, look up the price of your apples. Second, they can identify what kind of produce it is and how it was grown.

The numbers themselves are assigned by the International Federation for Produce Standards (IFPS). Every PLU technically has 5 digits, the first of which is usually a 0, indicating the food was conventionally grown, or a 9, if it’s organic. But many growers omit the first digit from PLU stickers, which can make it hard to tell.

Very rarely, you might see a PLU starting with an 8, which indicates that the produce was genetically modified. While many foods on supermarket shelves are made with genetically modified (GM) ingredients, the only fresh GM produce items sold widely in the U.S. today are corn, soybeans, papaya, and squash.

These stickers are useful for growers and supermarkets, but they aren’t mandatory. Not everyone uses them, and they don’t appear on all fresh foods.

“One can't imagine green beans or mushrooms with stickers,” the IFPS notes on its Frequently Asked Questions page [PDF], “however PLU codes may be used on signage or by another means for these types of commodities.”

If buying organic or non-GM produce is important to you and you’re looking at a 4-digit number, don’t fret—there are other ways to figure it out. Look for foods that are labeled “Organic” or have been certified as 100 percent organic by the USDA. “GMO-free” and “non-GMO” labels are less reliable, as those terms have not yet been regulated here in the U.S.

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