Answer:
Humans have two that shrink in the light and expand in the dark
Watches students at the prom to make sure there's no funny business.
A squiggly line tells you when you've done it wrong
What has been around for millions of years but is never more than a month old?
We traveled the sea far and wide. At one time, two of my sailors were standing on opposite sides of the ship. One was looking west and the other one east. And at the same time, they could see each other clearly. How can that be possible?
Black we are and much admired. Many seek us if they are tired. We tire the horse, and comfort man, and turn White when we've fulfilled your plan.