Motocross is a kind of motorcycle racing, but not on a road and not on a smooth track. The riders race on rough ground outside — dirt, mud, sand, grass — going up and down hills, over big bumps that throw the bike into the air, and sometimes through shallow water. The bikes are built for this: light, with knobbly tyres and long soft springs so they can land hard without breaking. The riders wear thick boots, body armour, and a helmet with a long front piece and goggles, because they are going to fall. Everyone falls. That is part of the sport. The word is both the name of the sport and a word you put in front of other words: a motocross bike, a motocross track, a motocross rider. In everyday talk in English-speaking countries, people often shorten it to 'MX' in writing, especially on race posters and bike names — a 'motocross bike' becomes an 'MX bike'. There is also a version raced indoors in a stadium with a man-made dirt course; that has its own name, supercross, though ordinary people mix the two words up all the time. One thing to notice: motocross is a race, with a start line and a finish. If someone is just riding a dirt bike in the hills for fun, that is not motocross, even on the same bike — English speakers would call that trail riding or just riding dirt bikes.