An old guy in a hardware store told me to always carry a 6 inch level

I was picking up some adhesive at the place on 5th Street about ten years ago. This retired installer, must have been 80, saw me looking at levels and just said, 'Kid, the big ones are for the truck. The little one goes in your back pocket every single job.' He tapped his own worn-out jeans. I've carried one ever since. What's the one piece of advice that stuck with you from a veteran?