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Chicken eggs take 21 days to hatch, so it's important to know the exact day you put them in the incubator. Also, keep track of the humidity and temperature readings. Turn the eggs one quarter to half a turn three times daily for the first 18 days. You want to turn then so that one side faces down and the other up. Mark one side of each egg with "X" and the other side with an "O" to keep track of which side is facing up. Candling allows you to detect infertile and bad eggs. It involves holding an egg against a bright light in a dark room to see inside. You can purchase a candling device, but for most situations, a small, bright flashlight will do. If you find any bad or infertile eggs, remove them from the incubator.  If you use a flashlight, its lens should be the small enough so that the light is directed at the egg. Another way to make a homemade candler  is to insert a desk lamp inside a cardboard box with a small round hole cut at the top. Put the egg in this hole to candle it. You may have to gently turn the egg up and down or from side to side to better see its contents. A living embryo appears as a dark spot with blood vessels radiating out from it. A dead embryo can show up as a ring or a streak of blood inside the shell. Infertile eggs light up bright and even since there is no embryo inside. On the 21st day, the chicks will "pip" their shells in order to breathe after bursting the air sacks. Watch them carefully after this point. It can take up to twelve hours after "pipping" for a chick to fully emerge from its shell. If some of the chicks haven't concluded their hatching after twelve hours, go ahead and remove the tops of those eggs.

Summary:
Keep track of time and vital statistics. Rotate the eggs. Candle the eggs after the first week. Listen for the sounds of the chicks starting to hatch.