School

I work in a little bike shop that specializes in vintage bicycles. We usually don't have many customers early in the day. But one day there was a guy who walked in right at 10 A.M. when I opened the door for business. He purchased a set of tubes, some chain lube and a few other things. The bill came to $13.59 including the sales taxes. He opened his wallet and immediately began apologizing. He said " Oh man, all I have is a 50-dollar-bill" implying that my cheesy little bike shop probably couldn't make change this early in the morning. " No sweat" I said as I took the bill from his hands. I was a little suspicious, however, so I checked his $50 over carefully. It was real. As I gave him his change, we noticed something that made us both smile and say, " Hey, that's kind of cool." The question is, what did we both notice?