Eggs have a shell that is made of calcium carbonate (CaCO3) crystals. The eggshell
1) is porous (see the 1-Minute NomNom "Get fresh")
2) has membranes that can contract upon cooling to create an important air bubble (see the 1-Minute NomNom "Eggs-tra ap-peeling")
3) allows unhatched eggs to breathe (because of the air bubble) (see the 1-Minute NomNom "Chicks love Air Supply".
To see how amazing these egg membranes are, take a look at this video below! The eggshell has been dissolved with acetic acid (vinegar), leaving us with a "membranes-only" egg that we can use as a toy! Talk about eggs-tra bouncy!
Second Helpings
Watch how strong the membranes are in this second video below where we see how "stretchy" they are despite being so "thin"!