Someone sent me a URL with this puzzle. It's attributed to Einstein, who claimed that only 2% of the population could solve it. I seriously doubt that Einstein had anything to do with it, and I solved it so it can't be that hard! I tried to look into the history, but Wikipedia also attributes it to Einstein - oh well.

It took me about 35mins, so give it a go and drop me a message if you beat my time! :D