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If you squint hard enough, the capital letter pi (Π) looks like half a circle.
(discuss.tchncs.de)
A "Showerthought" is a simple term used to describe the thoughts that pop into your head while you're doing everyday things like taking a shower, driving, or just daydreaming. The best ones are thoughts that many people can relate to and they find something funny or interesting in regular stuff.
That's not half the circumference then is it???
Now you're just being silly.