A mouth guard is a piece of soft plastic shaped to fit inside the mouth, protecting the lips, cheeks, tongue and teeth and the jaw when they are hit by large object - like a ball, or someone's elbow or head. Mouth guards help prevent injury to the mouth, teeth, lips, cheeks and tongue. Whether they also protect against concussion is unclear. What is clear is that mouth guards are an important piece of safety equipment that should not be overlooked.