What is the structure and function of the epiglottis?

The epiglottis guards the entrance of the glottis, the opening between the vocal folds. It is normally pointed upward while one is breathing with its underside functioning as part of the pharynx, but while one is swallowing, elevation of the hyoid bone draws the larynx upward; as a result, the epiglottis folds down to a more horizontal position, with its superior side functioning as part of the pharynx.

In this manner it prevents food from going into the trachea and instead directs it to the esophagus, which is posterior.