My dad told me about this a couple of months ago. Now I always adjust my car's sideview mirrors this way. From Car & Driver via Lifehacker.
Mark Frauenfelder is the founder of Boing Boing and the editor-in-chief of MAKE and Cool Tools. Twitter: @frauenfelder. His new book is Maker Dad: Lunch Box Guitars, Antigravity Jars, and 22 Other Incredibly Cool Father-Daughter DIY Projects