You can, you just don't. Carnival sword swallowers have been training apprentices in the craft for years and years. It's just technique. Start with your toothbrush and touch your toungue and mouth until you are on the verge of beginning to gag and then fight the urge to allow the gag reflex taking over. Fast progress depends on remaining calm while suppressing the reflex's initiation. I've seen carny sword swallowers who could put their fist down their own throats, bulging them out grotesquely. It's amazing what the human body can do if one has the will and the inclination.