Knowing this exchange rate would help you in moments of doubt: $1 = 4,000 Riel. Remember that when a mischievous ten-year-old monk charges you a dollar to use the toilet that costs 500 Riel per use after you tell him you don't have carry the currency.
What makes a person happy? Knowing that the happy horse is real and not just a figment of one's imagination. Also, having proof that contrary to what one's friends say, the existence of the happy horse is not just the result of too much alcohol intake. See? Happiness. =)