We're all told as kids that water in the northern hemisphere flows counterclockwise, whereas in the southern hemisphere it flows clockwise. This, people often say, is due to something called the Coriolis effect. But as you get older, some will then tell you that this is bunk, and it depends entirely on the the shape of the bowl. So, who's right? Does water in the northern hemisphere really swirl in the opposite direction to that in the southern hemisphere?