What is the difference between a mountain bike and a classical city bike? I'm talking about the anatomy of the bike, of course.
Mountain-bikes have wider and thiker tires that enable them to pass difficult terrain, but also slow them down. Their body is rougher, which also means they are heavier than city-bikes = reduced speed and you use more energy to get them going... Some more sophisticated mountain-bikes have shock-observers and springs in their body, similar to 4x4 cars.