Light travels from a light source. Your eyes are not a light source - though you might think they are. You'll get nowhere in a dark cellar without a light source - like a handy torch.

When the light from a light source is blocked, shadows are formed. The material that blocks the light is opaque - it won't let light through.

Shadows change throughout the day because the Sun seems to move across the sky. (In fact it's us that's moving, but never mind that now.) Shadows in the morning and evening are longer than in the middle of the day because the Sun is lower in the sky.

Shadows sometimes have a dark centre and a lighter area around them. This is because some of the light passes round - not through - the object to form a lighter shadow.

Even transparent objects can throw a slight shadow. The object stops some light, and so you can see it in its shadow.