When a swimming pool has had too much chlorine put in it and it REALLY stings your eyes.
geek: man this swimming pool really hurts my eyes
nerd: it must be overchlorinated
A geeky way that maths nerds express doing a 180 degree turn. (180 degrees is pi in radians)
geek: oh dear we just passed my favourite book shop (turns around)
nerd: looks like your going pi