Candling is a technique which is used to study embryonic development in eggs, and to assess eggs for commercial viability. People have been practicing candling for centuries, as the name implies: originally, the only piece of technology used in candling was a candle, although today bright sources of artificial light are used. Candling is very easy, and it can be very interesting, even with plain store bought eggs.
When an egg is candled, it is taken into a dark room and placed in front of a very bright light source. The light source allows people to visualize the structures inside the egg, which may be brought into even sharper relief by gentle jostling which pushes the yolk up against the edge of the shell, making it even easier to see.
During the candling process, it is possible to see flaws in the egg, such as blood spots, and people can determine whether or not the egg has been fertilized. If the egg has been fertilized and it is being incubated, it is possible to follow the steps of embryonic development in the egg by candling it on a frequent basis. As the chicken develops inside the egg, a spiderweb of veins will slowly appear, and the shape of the chick will start to emerge, like a very slow-developing photograph.
When eggs are prepared for sale, they are often candled to ensure that they are good to eat. Eggs with flaws or signs of being rotten will be discarded, while healthy, fresh eggs will be packaged for sale. Since some people object to fertilized eggs, farmers with free range mixed broods may candle eggs to separate fertilized from unfertilized eggs for sale, and farmers may also collect fertilized eggs to hatch into new chickens.
Although candling is a lot more fun when the egg is fertile and an embryo is developing inside, it's pretty neat with unfertilized eggs, too. If you have a bright lamp or flashlight, some eggs, and a dark room, you can try out candling for yourself. It usually takes a few tries to get used to the experience and to understand what you are seeing when you candle, and you may find a visual guide helpful. Several organizations including the United States Department of Agriculture issue candling guides with clearly labeled photographs illustrating the structures inside eggs.