Most birds and reptiles have an egg tooth, which is an essential part in the hatching process. On birds, it is small, sharp, and sits at the tip of the upper beak.
The egg tooth is the cream colored pointy object at the end of the beak of this chick:
While chicks are developing they need oxygen. Since egg shells are porous, chicks receive oxygen through the pores. As hatching day nears, though, the chick gets too large for oxygen to flow well. It is then that chicks use their egg tooth to break into the air cell inside the egg to get more oxygen.
There is a limited amount of oxygen available in the air cell, so soon they use the egg tooth to pip a small hole in the outer shell. Then they rotate and unzip the egg shell.
In the cracked part of the shell the beak and egg tooth are visible in this photo:
White Leghorn chick pushing itself out of the unzipped egg shell.
This Easter Egger is resting after a long day.
This 2 day old chick still has its egg tooth, but within the next day or two it will fall off.