Economics

I am walking down a busy sidewalk and I badly need to pee. I remember that at the end of that narrow alley on the right there is a public toilet. I rush. When I get there I am reminded that to unlock the toilet gate you need a 10 baht coin. I don't have any. There is no one around who can give me change.
My nearly exploding bladder is enhancing my resourcefulness: I have an unexpected idea. I remember seeing a nice lady with a radiant smile squatting on the floor at the alley entrance, begging the passersby for some spare baht. Hadn't I been distracted by my body needs I would have already dropped a coin in her bowl. I grab a 20b note out of my pocket. For most people this note is worth double a 10b coin. And that, of course, includes the lady with the lovely smile. However, I want to pee now, and I need the 10b coin, which I don't currently possess, and that makes it priceless for me, no matter what's worth for you or anyone else.