Contact sports are sports that emphasize or require physical contact between players. Some sports, such as mixed martial arts, are scored on impacting an opponent, while others, including rugby football, require tackling of players. These sports are often known as full-contact, as the sport cannot be undertaken without contact. Other sports have contact, but such events are illegal under the rules of the game or are accidental and do not form part of the sport. The contact in contact sports can also include impact via a piece of sporting equipment, such as being struck by a hockey stick or football. Non-contact sports are those where participants should have no possible means of impact, such as sprinting, swimming, darts or snooker, where players use separate lanes or take turns of play. Consideration should also be given to other sports such as Moto-cross and Bicycle Moto-cross (BMX) and Cycling which all involve riding/racing in packs of riders. This often results in brushing and bumping off other riders. It can additionally result in crashing, and possible head injury. Even though these riders wear helmets, head injuries can be serious.