A fist is an action where a hand has the fingers curled into the palm and the thumb retracted, displaying the knuckles. There can be an open fist or a closed fist. Formation of a fist for the purpose of punching is the most basic knowledge taught in the sport of boxing. Fists are taught in martial arts like karate, kung fu, and taekwondo for the process of punching and striking. Fight 100% says "more than 90% of the people actually don't know how to make a fist correctly", and adds that correct formation means the user "won't break [their] hand", "won't strain their wrist", will "be able to launch very powerful punches", and "be able to knock someone out in one punch". Fists are also well-known revolution symbols—as "persistent symbol[s] of resistance and unity", in the form of a raised fist. Various phenomema, which include the term "fist" in their name such as the sexual act of fisting or the fist bump greeting, involve the use of a fist. Improper formation of the fist whilst punching or striking an object can cause bruising and broken small bones in the hand known as Boxer's Fracture. Boxer's Fracture occurs when metacarpals or small bones in the hand break on the side of the pinky and ring finger. The name derives from the fact that such injuries are most common in boxers and practitioners of other fighting arts. Making a fist is virtually unknown amongst other primates. This is because while "most primate hands are long of palm and finger [and] short of thumb", the proportions are the opposite for humans. At least one study has claimed that the clenching of one's fist can be used to recall information. Some studies have shown that making fists can help humans to cope with stress or anxiety because the mind gets preoccupied with the tightening of the muscle to focus on the issue at hand. The act of creating a fist is known as "making a fist" or "clenching a fist".