Hatching and breeding your own chicken was a common practice in the good ole days. We bought fertilized eggs from special vendors, and then incubated these at home using D-I-Y incubators. But not all of those eggs would turn out to be good.
For the first few days, we would use a torchlight to shine through the eggs and look closely for evidence of amorous fowl play — a dark spot, which would be the embryonic chick. The absence of this would mean the egg had not been fertilized – such would be picked out and turned into omelette. LOLZ.
This process was called “candling” — a legacy term from the days of using a candle light.
Kids today would rarely know and have the opportunity to appreciate the fun – of separating the dudes from the duds. Something to chuckle about.