My Buff Orpington hen, Ginger, is the broodiest hen I've ever had. She is only a year and 3 months old and already she has been broody about 4 times! So, I decided instead of trying to fight her on it, I'd let her set on a few fertile eggs I'd buy from a local breeder. I got the eggs, put them under her and she diligently kept her post for 21 days until I noticed the beginning of pipping on one of the eggs I could see. A small hole was starting to form on one egg so I peeked under her feathers at the other egg, and sure enough that one was pipping also.
Baby chicks are born with what is referred to as an "egg tooth" which is used to crack the eggshell so they can work their way out of the egg. The action of poking these holes is called pipping. The chick will crack the eggshell all the way around the blunt end of the egg until it is loosened and then it will push the shell open with its feet and out it comes. Welcome to the world little babies!