Why do my friend's glasses make things look blurry when I put them on? Yesterday I tried on my friend's glasses and noticed that they made everything look blurrier and farther away. If they do this to my vision, how can they make things look clearer and closer for him?
 A: Glasses are meant to correct defective vision. In some sense your friend's eyes have "somebody else's glasses" on all the time, so the world looks blurry to him. The glasses he wears "undo" the problem of the other glasses and make the world clear again.
The strength of glasses is sometimes expressed in diopters. Perhaps your friend has eyes that have a +2 "error". By wearing -2 glasses, he is back at zero error. But if you, with your good eyes, put on the -2 glasses, you end up with a -2 error, and blurry vision.
This can be explained with detailed physics - but from the question I expect you are looking for this qualitative answer. Please comment if you need more detail.
Some nice diagrams are given on the hyperphysics site - from which I reproduce just one:

To see things in focus (not blurry), the lines that represent the rays of light have to intersect at the retina (the light sensitive surface on the back of the eye). A good lens will do this - a defective lens will either focus in front of the retina, or behind it. An additional lens puts the intersection at the right place. But if you are "correcting" for a mistake that isn't there, you create a problem that wasn't there.
A: The above answer is good. Thought I would just give a more visual explanation for you. Blurry vision is usually caused by the focal point being either before or after the retina. Glasses move the focal point. If your focal point is correct, putting on your friends glasses moves your focal point to the wrong place, causing blurriness.

Image from Bing Search: Originally found on http://www.slipups.com but has since been removed from that site.
