Why do hard boiled eggs have void under shell and why is it always on the fat end?

You're right, there is often a void under the shell of a hard-boiled egg, and it's usually located at the wider, fat end of the egg. Here's why:

Air Sac:

* Function in Raw Eggs: All eggs have an air sac at the fat end. This air sac is filled with air that the chick breathes during incubation. It gets larger as the chick develops and needs more air.

* Boiling and Expansion: When you boil an egg, the heat causes the liquid inside to expand. This expansion pushes the air sac to the fat end, where it's less confined by the egg's shape. The pressure can also cause the shell to expand slightly, creating a larger void.

Why it's Usually at the Fat End:

* Shape of the Egg: The fat end of the egg is naturally wider and has a less rigid shell than the narrow end. This makes it the easiest area for the air sac to be pushed towards and for the shell to expand.

* Position During Cooking: The fat end is typically the top side of the egg during cooking. This allows the air sac to rise to the top, making it easier for the void to form at the wider end.

Note: The size of the void can vary depending on factors like the freshness of the egg, the cooking time, and the temperature of the water.