How Big is 10 Inches?

I left my tape measure at home today. I'm working on a sweater that I'm hoping to finish this month, if I can keep to a skein-a-day schedule. (I'm still on Tuesday's skein, but I have about three more hours of lecture to knit through today and some reading.) Right now, I need to knit for 10" and then start the hip increases. How do I know when I've hit 10"?
 

Luckily, my foot is 10"! Even better, there's almost no one left in the office to see me pulling off my shoe
to use my foot to measure the sweater I'm knitting. Crazy? I prefer "innovative."