A child who was not there
The puzzle
They took a photograph, and a child appears in it who could not have been there.
See the answer (spoiler)
Spoilers from here on. If you want to work it out yourself, leave this folded and move on.
Answer
It was taken in panorama mode.
The child was at one edge, walked round behind the camera while it was still sweeping, and was captured a second time at the other edge.
