When the shutter opens it lets light into the camera. Just for a split second, maybe less. The lens focuses the incoming light onto a small square of plastic that has been treated with chemicals, which is called film. The light interacts with the chemicals and leaves an image. If another drop of light hits the film before it is processed, it will be ruined forever.
In a darkroom, you essentially reverse the process: now you blast light through the film, and onto a piece of paper which has also been treated with chemicals. You then take this paper and dunk it in several tubs of poison. You need to get the order of poisons right, and the timing, too. Otherwise, the picture will be ruined forever.
And after all that, you have a photograph.
But sometimes, just sometimes, a hair will fall into the works and be caught in the negative and live forever as a spectral addition to the picture, a thin and unerasable reminder that human beings make art with their hands.
How many tambourines does one man need?