They are FREE!!!

In my modest 1100 sq ft photo studio space, I have a small fridge.  It is big enough to hold some water bottles, cokes/sodas, beers, etc, and stands about waist high.  I have it in the studio for clients more than for me.  Anything is the fridge is there for the taking when a client walks in, no need to even ask.  

Somehow, though, the fridge keeps growing dollar bills...  Yup, I opened it up one night and there was a dollar in there.  Then two.  Now three.  I don't understand why people feel the need to do that when I specifically tell them, "Feel free to take whatever you want."  

It would be different if I added, "Just be sure to leave a buck or two", but people do it anyway.  I'm not sure why, but they do.  Maybe I'm in the wrong business?  Maybe I need to open up a vending machine company?!?