When your whole back shudders for a second and you kind of shake all over your body, and your mum might say "Oh, someone walked over your grave" which is a bit morbid - why does all that happen? Is there a use for it?
Just shook someone's hand and been doing it periodically every minute or so. Brr.

Feel free to ask some unrelated biology questions below if this doesn't float your boats but this isn't (necessarily) a filth thread.