Free Rice is a website where you play a vocabulary game while ads are displayed at the bottom of the page.  For each correct answer you get in the game, 20 grains of rice are "won." The rice is paid for using revenue from the advertisements appearing at the bottom of the screen. 

I'll admit, I have to use some of those multiple "guess" reasoning skills I learned in high school to get some of the answers.  Give it a try when you have some time to spare.