Light! Without it, the world would hardly be worth living in.
Light comes to us from the Sun, of course, and other luminous sources, and some materials let it through them better than others.
Transparent materials, like glass, clear plastics, water, etc., let most of the light through them.
Translucent materials, like tracing paper, jelly, frosted glass, etc. let some of the light through.
Opaque materials, like metals, wood, rock, etc. let no light through.
If light is shining on an opaque material, which doesn't allow light through it, then the light is blocked and a shadow is formed, which is the same shape as the opaque object blocking the light - your body, for example!
So, let's have a look at what's going on with light....