Mouth guards work by forming a protective layer over your teeth while playing contact sports. That way, your teeth will still be intact if you receive a blow to the face. Don't let your game time get cut short by an emergency trip to a dentist.  Mouth guards should ideally be replaced after each season because they can wear down over time, making them less effective. Replacement is especially important for adolescents because their mouths continue to grow and teeth continue to develop into adulthood.