-21
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.
I can level up with more sources (stanford, wiki).
The circumference is 2pi x radius, which obviously means half the circumference is pi x radius.
Edit: formatting. Asterisk results in italics
https://en.m.wikipedia.org/wiki/Pi