Shape of the Earth
One of the very stupid things you learn in school is about the shape of the Earth. You are told that the Earth is not exactly spherical - it has a "geoid" shape (which basically means it has a shape like the Earth... very intelligent). Like in the Microsoft joke , this is technically true, but rather pointless. The Earth's radius is 6500km; the deepest trench in the ocean is 11km and the highest mountain is almost 9km. Which makes it a pretty damn good sphere, with a precision of about 1:600. Think about it this way: the same precision on a billiard ball with a radius of 3cm would mean a maximum error of 0.05mm, or 50 microns. I'd call that a perfect sphere...