Down The Plughole

The interesting thing to do today - according to 3yo M -was to watch water going down the plughole - always swirling away in the same direction.

My reputation as the Granny who knows the answer to everything - (didn't really expect grandchildren to believe this) - has taken a knock - if not gone down the plughole - because I don't know why this happens

Answers welcome - hopefully you paid more attention in science lessons at school than I did.