That if your teenager is going on a field trip to the mall, and all you have is $2 or a $20, you give him the $20.
It is further guaranteed that if he is told 'you MUST bring back change', that he will not bring a single cent.
But if it is my kid, he'll spend the $20 on a video game and have no money to buy the lunch that was the purpose for going to the mall in the first place.
Also a given with my kid, he'll buy it from the vintage gaming store for double what it would have cost if dear old mom looked for it on eBay.
Lesson learned. And next month's trip-he only gets a gift card for the restaurant they'll be visiting!
Addendum: It is also a safe bet that the teenager will come to you the next day to ask to buy a $10 app, which you later explain if he hadn't spent $20 on a game found on eBay for $6.00, you might be more inclined to buy it. Message received.