CoachA light source is a thing that makes its own light: the Sun, a lamp, a candle flame, a firefly. Everything else — a book, a ball, the walls, you — makes no light of its own: we only see these things when light from a source falls on them.
Examples
Example 3.2 (Sources or not?)
The Sun, a torch, a burning match and a television screen all make light: sources. The Moon, a mirror and a white wall can look very bright, but they make no light of their own — they only send back light that reaches them. Switch the lamp off, and the brightest mirror goes dark.
Example 3.3 (Try it: the dark room)
In the evening, close the shutters or curtains of a room, switch off every lamp, and wait while you count slowly to . What do you see? At first, almost nothing — with no source, there is no light to see by. Then open the door a crack: one thin slice of light is enough to bring back tables, chairs and toys.
Example 3.7 (Try it: shadow animals)
With the lamp of Method 3.6, put your hands in the light instead of a toy: two fingers up make rabbit ears, both thumbs crossed make a flying bird. The wall shows the shape your hands cut out of the light — a shadow always keeps the shape of its blocker, seen from the lamp.