Skid marks are lines or streaks left on the ground by tires on a car or bike that have moved suddenly and in an uncontrolled way. They are usually caused by the tires skidding on the ground when the brakes are applied quickly - for example, when a car or bike has to slow down quickly to avoid hitting an obstacle or another car. If the skidding lasts for too long, the rubber from the tires can be left behind on the ground as a mark.