The
instinctive reaction your body undergoes when a
foreign body (i.e., anything that isn't usually supposed to be there) approaches your
eye.
Wanna see this one in action?
- Point your finger.
- Poke it toward your eye.
If your finger is suddenly obscured by your eyelid snapping shut, you've experienced the blink reflex.
If you can now feel your cornea with your fingertip, remove your finger and seek medical attention.
The blink reflex is what keeps dirt, rocks, bugs, and stray fingers from touching your eyeballs. It's a good thing.
Do not attempt to retrain your body to relax, as some opponents of the gag reflex have done with their personal pet peeve. The familiar maxim regarding fun to be had and eventual occular loss applies.