This is Explainer, a column that answers questions we all have (or should have). Anyone with skin knows the feeling of having a bad itch that won’t go away. Whether it’s a mosquito bite, hives, or just a mystery lump, you can’t stop scratching. It may feel good in the moment, but the feeling won’t go away. It’s a perceptual cycle of itch-and-scratch, but why? Itchiness itself can come from a number of things. Typical culprits include bug bites, allergic reactions, skin conditions, even itchy sweaters. Sometimes, itches can tell us something important: Brian S. Kim, a professor of dermatology...