People trade currencies all the time, but how can currency be an investment? Here's a simple example. Imagine that you took a trip from the United States to Europe in 2002. For the trip, you changed your US dollars into euros. At the end of a trip, you typically would change any extra euros back into US dollars. But what if you didn't?
In 2002, one euro was worth about 90 US cents ($0.90). Say that you decided to hold on to 500 euros, and left them sitting in your desk drawer for 5 years. In 2007, you took your euros to the bank and sold them for a 2007 price of $1.40. Since you bought the euros for $0.90 and sold them for $1.40, you made a $0.50 profit per euro. You would have made $250 just because you held on to those euros and had bought and sold at the right time. That's a 55% return in 5 years.