It's not necessary to take a bite of sour food, or even get it all the way into your mouth, to react to it physically. It is such a sharp flavor that just touching something sour to the tip of the tongue can be enough to set off the visible reaction. The response is akin to a whole face squint -- the brow furrows, eyes narrow or squeeze shut, the nose crinkles and, most noticeably, the lips pucker into a round, tight little mass.
