Most of us grew up believing that gravity is just the mass of two objects attracting each other. We probably still think that, really, since who spends their time reading up on gravity? Well, that's WRONG.
Our old friend MinutePhysics explains that gravity is actually based on energy and momentum, which means even light has gravity (yeah yeah, extra credit if you already knew that). So basically: Suck it, Newton.
And also? All of your word problems in sixth grade math class were wrong, because they didn't account for the curvature of the Earth. [MinutePhysics]