Why does dust in my eye hurt, but my contacts don't?

Matt:

Why does it hurt when I get a speck of dirt in my eye but when I put my contact lenses in, I don't feel much of anything?

-- Eyes, San Diego

The human cornea is choosey about what it reacts to. Tiny things like dust or dirt hurt like heck and potentially could do damage to your eye. Large, smooth objects are pretty much ignored. Notice that you don't feel anything every time you close your large, smooth eyelids. If we weren't wired up this way, we'd probably walk around all day going blink, ouch, blink, ouch....