Mouth guards and Multiplication

Matthew's aunt gave him twenty dollars for his birthday.  "I've always wanted money!" he cried.  Talk soon turned to what he could buy with twenty dollars and, as always, Matthew has pretty clear interests.  "Can I get a real hockey stick with twenty dollars?" he asked.

"No," I answered.

"Can I get a mouth guard?"  This has been Matthew's dream for about a year now.  A mouth guard. 

"I'm not sure how much a mouth guard costs," I told him.

"Get your computer!  Look it up!" he insisted.

A few keystrokes later, "Looks like a mouth guard is about ten dollars," I said.

"Oh NO!" Matthew moaned.  "I wish I had ten dollars!"

At this point Cameron broke in.  "Matthew.  You DO have ten dollars.  Twenty dollars is like ten dollars TWO times.  You have TWO ten dollars!"

Matthew's eyes widened and he dropped his voice, leaning in to Cameron.  "Hey.  You want me to buy you a mouth guard too?"

Cameron kindly suggested that Matthew could use the extra ten dollars to buy something else.  And now Matthew is the proud owner of his very own mouth guard and Froggy Rides a Bike by Jonathan London.