A couple of weeks ago I made a purchase at the local farmer's market. The total was $15.34. I handed over a twenty and counted out 34 cents in change.
The young (I am guessing high-school age) fellow who took the money looked at my offering a bit strangely and said "oh, it's under 20, you don't need to do that" (meaning giving him the 34 cents).
I countered with a quizzical look of my own and said, "But if I do, then you can just give me a five back, right?"
After a quick double-take, he said, "Oh! Well, if you want to do it that way." I did, so he did, and we parted on good terms.
The young man seemed to have no cognizance of the convenience to me of not getting 4 ones and a mess of change back; nor of the convenience to him of not running out of 1-dollar bills during the day when others might expect him to make change.
I don't think he was dumb. It's a cultural/generational thing. I'm sure the cashier has never in his life paid for anything in cash. Friends of mine who have kids about his age say it's true - they charge every purchase, even if it's less than a dollar. No wonder that they have no experience with receiving, or giving, change.