When I got stationed in Japan I decided to teach my son the difference between money (buying power) and currency (dollar, yen, pound, etc.). He was getting a certain amount every week. When my wife and son arrived (I didn't bring them over until I had secured housing) I pegged the equivalent yen amount to the exchange rate for when they arrived. Then each week he got to decide which currency he wanted to receive his allowance in. The dollar and yen amounts were fixed despite the then current exchange rate. Example: Say the exchange rate was 200 yen to the dollar and his allowance was originally $5.00/week. That means that he could choose to get either 1,000 yen or $5.00. I don't recall the actual amounts in play but those are good round numbers.
Chase, the bank on base, would let us exchange dollars to yen and vice versa for a tiny fee. Now back then the ratio between dollars and yen could vary greatly in the matter of a week or two. There were actually times when the exchange rate was so favorable towards yen (I saw it approaching 100 yen to the dollar) that if he elected to get allowance in yen, it cost more dollars for the originally set yen amount. At 150 yen/dollar, if he chose yen that week, the 1,000 yen would cost ME $6.68. Less the bank's small fee, he would net $6.50 if he converted the 1,000 yen to dollars or a 30% temporary raise! Conversely when yen was cheap - say 275 yen/dollar - his 1,000 yen only cost me $3.64. The magic was in converting from one currency to the other AND BACK.
Take for example when the yen rate was 275. If he got his allowance in dollars he'd get the set $5.00 which he could take to the bank and buy yen at 260 (bankers always get their cut off the top) and come out with 1,300 yen. If he held it until the yen got expensive again - 150 net after fee - and his 1,300 yen became $8.67! Hey! The 11-year-old is making money without working anything but his noodle. It's always better to work smarter than harder. We got the "Stars and Stripes" newspaper delivered daily and he always wanted the section that included exchange rates on Friday morning so he could decide how he wanted his allowance that week. It also encouraged saving since holding on to his money could potentially reap him a bigger reward.
My son had serious problems due to aphasia but he could do basic arithmetic in his head.
BTW, the bank allowed us to have both a dollar account and a yen account and to transfer between accounts at the going rate almost for free. If the treasurers of the various on-base clubs/organization had their act together at all they were growing their group's balance like this continually.