Doctors have said this for a long time: don't scratch an itch. Scratching feels good, so why is it bad for you? Now a new study explains why. Dr. Daniel Kaplan studies skin problems at the University of Pittsburgh in the U.S. His team put something on the ears of mice that caused an itch. The researchers let some mice scratch, but stopped others from scratching.

The mice that scratched had more redness, and their ears swelled more. The mice that could not scratch had a much smaller rash. This shows that scratching makes a rash worse.

Kaplan said this matches what people feel every day. If you don't scratch a mosquito bite, the itch usually stops in five or ten minutes. "But if you start scratching it, it's your friend for a week," he said. The itch gets worse and lasts longer. Kaplan's advice is simple: try not to scratch, even though it is hard.