Why Do Our Eyes Sparkle When We Are Merry?
If you should watch very closely the eyes of a merry person when you see them sparkle you would probably notice that the eyelids move up and down more often under such conditions than ordinarily, and if you know what moving the eyelids up and down in front of the pupil of the eye does, you will have your answer.
Every time the eyelid comes down it releases a little tear, which spreads over the eyeball and washes it clean and bright. It does this every time the eyelid comes down. Now, there is something about being merry which has the effect of making the eyelids dance up and down, and thus, every time the lid comes down, the ball of the eye is washed clean and bright, and gives it the appearance of sparkling, as we say.