Why are honeycombs hexagon shaped?

Answer 1

Because a hexagon shape makes the honeycomb very strong, so less prone to damage, but how the bees know that is a mystery.
It is the most efficient use of space and gives the greatest number of cells for a given amount of wax.

Actually, if you look more closely at honeycomb the cell itself is more or less circular. It is just that they are arranged in a hexagonal pattern.