Why does one start making faces or shut the eyes while eating sour foods like imli, lemon, etc.?

Explanation

This is due to a protective reflex action that our body performs because of intense stimulus like sourness. Ordinary stimuli will not evoke such a reflex action. Some of the nerve fibres convey the taste to the brain. These stimulate the neurons (nerve cells) of the facial nerve that controls the muscles of the face. The very sour stimulus leads to a reflex contraction of all facial muscles, screwing up of the entire face with eyes shut tightly. The response is temporary and we may either enjoy the taste or intensely hate the taste.