Why do I cringe when someone scratches their nails down a chalkboard?

It has been hypothesised that it evokes the sound of babies crying, but it's really not so much the sound itself as the thought of scratching our own nails down a blackboard. This is quite a painful thing to do because the vibrations are transmitted directly to the nail-bed and we may just be empathising with that sensation. Answered by Luis Villazon