One day, one of my roommates gave me what she said were some hundred-dollar bills to pay me back for money she had borrowed from me. I didn’t entirely trust her not to take advantage of me, so I later asked a coworker whom I thought I could trust if the bills were in fact hundreds, and he said that they were, so I went home and put them into my bureau until I could get to the bank to deposit them.
A few days later, I went to buy groceries with a bill that I thought was one of the hundreds, but the cashier told me that it was a one.
I still have no idea what happened. Maybe my coworker took the hundred and gave me back a one (I think this is unlikely, but technically I can’t rule it out). Maybe my roommate took the hundred out of my bureau and put a one in its place. Maybe the cashier was lying to me and ripping me off. I just don’t know and probably won’t ever know, but there would have been a lot fewer variables if I could tell a one from a one hundred myself.