Usually made out of silicon, mouth guards are designed to protect the user's teeth, lips, gums but all tongue from a powerful impact to the bucal area. Mouth guards are usually designed to protect the top row of teeth.
How does a mouth guard help keep athletes safe? First of all, it keeps the teeth in place, generally keeping them from getting knocked out. It also protects the gums from damage. If the user gets struck by an upper cut while his tongue is between his top on bottom teeth, the mouth guard will keep the teeth from chopping the tongue off as well as damage themselves. Finally, the mouth guard keeps the lips from exploding on the teeth if hit.