Smelly means having a strong, usually unpleasant odor. When something is smelly, it gives off a scent that makes you wrinkle your nose or want to move away. Gym socks left in a backpack for a week become smelly. A trash can on a hot summer day gets smelly. Your dog might be smelly after rolling in something mysterious at the park.

The word almost always suggests a bad smell, not a good one. You wouldn't call fresh cookies smelly even though they have a strong scent. Instead, you'd say they smell delicious or amazing. But that carton of milk that's been in the fridge too long? Definitely smelly.

Some things are naturally smelly, like certain cheeses or durian fruit. Others become smelly through neglect, like when you forget to take out the garbage. Scientists who study smells have discovered that humans can detect over one trillion different scents, and we're especially sensitive to smells that warn us about spoiled food or other dangers. That's why smelly things can make us feel uncomfortable: our noses are trying to protect us.