The currency exchange market offers you the chance to make money. Please note that currencies, otherwise called foreign exchange, often fluctuate in value on a daily, at times even hourly, basis. So, if you bought 100 euros today, for US$160 and tomorrow somebody buys it at US$180, then you made a profit of US$20 in just a day. Of course, there's also the risk that instead of increasing the value could decrease.
Because the exchange market changes daily. Different currencies will be worth more sometimes than at other times. By checking the exchange rate each day, you will know on what days to exchange your currency to make money.