It's been a long winter for many of us in the Midwestern United States, and the sun's long-awaited return is certainly welcomed by those of us saddled with cabin fever.  On days like this it's fun to do the old "magnifying glass on leaves" experiment, and while I don't advocate performing this on living things, I do think you should do this at least a few times in your life.  Most people don't really think about the physics behind focusing the sun's rays into a beam hot enough to set paper alight, but this video from Science Friday does a really good job of explaining it simply:

An interesting note from the video:  Many commercial magnifying glasses are able to focus the intensity of the sunshine 1000 times.  No wonder it sets paper on fire!  But what's more amazing is that the lens in the human eye is able to focus 2 1/2 times more than that.  Does it make sense now what your teacher said about not looking into an eclipse?

Magnified Sun Burns:  http://www.sciencefriday.com/videos/watch/10380