The structure that covers the trachea so that food does not enter the lungs is called the epiglottis. It is basically a flap of tissue that sits at the very top of the trachea at the very back of the tongue. During regular breathing, the epiglottis is open, thus allowing air to enter and exit the lungs via the trachea. But when a person swallows food or drink, the epiglottis folds down to cover the trachea and direct the food/liquid into the esophagus to enter the stomach. So it keeps food out of the trachea and lungs (windpipe). If something solid should make it past the epiglottis, a coughing reflex will kick in to propel the material back into the mouth.