It's bad news for your used paper cups.

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This story originally appeared on marthastewart.com by Jillian Kramer.

Recycling as much as possible sounds like a good idea, but you may be surprised to find out that many of the items you think belong in your recycling bin-soiled pizza boxes, paper coffee cups, and plastic utensils, for example-have no business being there. To help you treat the Earth right and spend less time guessing whether or not your items belong in the trash or the recycling bin, we're sharing the seven most common items people assume are recyclable, but actually aren't. Here, seven items that you can't recycle-or that you need to clean before you do.

Related: The Best Reusable Products for Outdoor BBQ Parties & Picnics

Pizza Boxes

While cardboard is recyclable, cardboard that has been stained with grease isn't recyclable-and that means most pizza boxes aren't eligible for recycling. Why? "The grease prevents the cardboard from being broken down and reprocessed to make other paper products," explains Gabby Petrelli, conservation action coordinator for Shedd Aquarium. With that being said, make sure to check the top of your pizza box. If it's clean (as most are), tear it off and add it to your recycling pile.

Related: Here's Your Guide to Recycling By State

Paper Coffee Cups

"Disposable paper coffee cups are coated in plastic, which is difficult to separate from the paper to allow it to be pulped and reprocessed, though it is possible," says Petrelli. (And where it is possible to separate, Petrelli says, it's often not done because it's so expensive.) So, rather than using paper coffee cups, switch to reusable mugs, Petrelli suggests, which can be filled at your local coffee shop or at home before you hit the road.

Opaque Plastic Cups

While your clear plastic cups are perfectly fine for the recycling pin, opaque plastic cups made from Plastic #6 aren't recyclable-because when Plastic #6 is heated, Petrelli explains, it can "release toxic chemicals that make it dangerous and expensive to process." If you use plastic cups regularly, switch to clear varieties in order to be able to add them to recycling.

Related: 14 Recycled Crafts That Will Help You Go Zero-Waste

Plastic Utensils

Like opaque plastic cups, most plastic utensils are made from Plastic #6, and pose the same problems at recycling facilities. What's more, "plastic utensils are often not individually labeled with their plastic type, making it difficult for [recycling facilities] to sort them properly," Petrelli explains. Stick with metal silverware, if possible.

Pumps and Nozzles from Product Dispensers

Your plastic soap bottle is recyclable-but the nozzle that pumps it into your hand isn't, says Petrelli. That's because those nozzles and pumps have metal inside them, preventing them from being melted down effectively.

Plastic Bags

Despite being made of plastic, plastic bags should not be added to recycling bins-or to the trash, for that matter. "People may think that anything called 'plastic' is recyclable, but that's not so," says Susan Barton, director of facilities at Shedd Aquarium, who explains that "recycling materials are sorted through a series of conveyor belts. Plastic bags end up wrapped around the machinery that keeps the conveyor belt moving. And when that happens, the entire line must be shut down to remove the bags." Plastic bags that are tossed into the trash end up in landfills, where they can "fly away into the landscape and into water systems, or disintegrate into smaller pieces," adds Barton. "Both results negatively affect wildlife and the environment." Instead, switch to a reusable bag when you go grocery shopping or hit the mall. "Or, if you must use a plastic bag, take them back to the stores that collect them for recycling."

Single-Use Plastic Straws

Plastic straws can technically be recycled, but because they are so small and thin, they often get tangled into recycling machinery-and therefore, not all recycling centers will accept them. Unfortunately, plastic straws are also "often one of the top 10 litter items found on beach cleanups," says Petrelli, so it's not a great idea to toss them in the trash, either. If you can, stick to reusable ones made from stainless steel or bamboo.

This article originally appeared on marthastewart.com

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