I've been playing this game since middle school, when I used it to study for the vocabulary sections of standardized tests. For every correct answer, Freerice donates 10 grains of rice to the United Nations through their World Food Program.
They've since added tons of categories, like math, foreign languages, biology, geography, and even SAT prep. It's a great way to flex your brain, learn some new things, and help a good cause all at the same time.
Bump up the level manually if the questions are too easy for you at the beginning. They'll automatically get more challenging as well.
PS. If you are going to play, disable your adblock for their domain - they generate revenue to pay for the rice with the ads they serve you at the bottom of the page. It's one of the domains I have whitelisted.
Ramp