MathRun

Here's a math quiz that starts out deceptively simple and slow, but gradually becomes more difficult while you're telling yourself how well you're doing. Before you know it, you're scrambling to keep up! I got to one point where I didn't even recognize the symbols, but figured it out somehow. I cannot tell you what happens when you get an answer wrong, because I didn't, but still ran out of time on level 8. Don't scroll down to check the high scores; they will just make you feel bad. Link -via mental_floss