A little boy learns why you shouldn't mess with the tooth fairy.

FOX's Krystin Goodwin explains:

Most kids love getting money from the Tooth Fairy. An 8-year-old boy in Texas was looking to make a little extra cash and wrote the fairy a letter...

It reads: "Dear Tooth Fairy, I want a $5 bill or higher," telling her "I worked so hard to pull my tooth out. Have the money under my pillow by afternoon."

The child got a response with a dose of reality. It reads: "I have been assigned to your case. One thing we do agree on and have taken into consideration, it was very bold of you to ask for more money. Since all your teeth have been cavity free, we decided to increase our TPO: Tooth Pay Out, by one dollar for future extractions... Should you place a cavity filled tooth under your pillow, you will receive a bill to cover our costs of flight travel."

The lesson in economics going viral on Reddit.

I'm Krystin Goodwin, FOX News.

Follow Krystin Goodwin on Twitter: @KrystinGoodwin