If you want to keep light out of your eyes, you always put your hand between them and the light, because you know the light goes straight and so will not go around your hand." "Yes, that's true, and when I want to make a shadow anywhere, I put something right before the light." "Certainly.

Well, the rays of the sun all come to us straight, and side by side.

They are pretty hot, but not hot enough to set fire to anything that way.

But if you can gather a good many of these rays together and make them all shine on one little spot, they will set fire to whatever they fall on.

Now a piece of glass or any other thing that you can see through easily,--that is, any _transparent_ thing, lets the sunlight through it, and if it is flat on both sides, it doesn't change the directions of the rays.