Two Eyes are Better Than One

1 cup
1 penny
1 friend
1) Put the cup on a table and stand 9 feet away.
2) Cover one eye. Have the friend hold the penny at arm's length above the cup, but slightly in front.
3) Watching only the cup and penny, tell your friend where to move her arm so that the penny will fall into the cup when dropped.
4) Tell him to drop the penny, then see how close you are. Explanation: Your eyes don't see things the same. They see the same thing but at a slightly different angle. Your brain takes both views and combines them into one 3-d image. This kind of vision is called stereoscopic vision. When you cover one eye, you no longer see with stereoscopic vision, instead you see things two-dimensional, like a photo. This makes it hard to judge distances. People who lose sight in one eye learn to cope, but it is harder.