Question

The two openings of the pharynx, one leading to trachea and the other leading to oesophagus, lie very close to each other. Yet food we swallow normally doesn't enter into our trachea. Why?

Answer 1

Because a valve is there to prevent the food from entering the trachea.

Answer 2

The food does not enter into trachea because during swallowing, the aperture leading to trachea  (glottis) gets covered by a cartilaginous flap called epiglottis and the food has no other passage except  going into the oesophagus.