If you’ve ever had to spend a few minutes waiting around with nothing but a bottle of water to amuse you, you’ve probably noticed a strange symbol appearing on the bottom: a triangle with a single-digit number.

The swishing-arrow design is, of course, the widely-recognized symbol for recycling. But what do those numbers inside the logo mean?

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Business Insider recently helped clarify things. The stamp is a Resin Identification Code, or RIC, and the numbers indicate exactly what type of plastic is being used for that particular container: “1,” for example, is polyethylene terephthalate, a linguistic nightmare of a plastic typically found in soda bottles and peanut butter jars. High-density polyethylene is designated “2,” a sturdier plastic that can accommodate heavier products like a gallon of milk or laundry detergent. Low-density polyethylene (“4”), in contrast, is pretty flimsy, and you’ll typically find it used for grocery bags or shrink-wrap.

The series goes up to “7,” which indicates a mixture of plastics being used. The codes help local recycling centers sort their returns and indicates to consumers which containers can be turned in. Not all centers are equipped to deal with every type of plastic, especially PVC, or “3” and “7,” which can be found in everything from sunglasses to bulletproof material.

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