Why are ceramics like this?

he interesting question is why ceramics behave like this—and the no-less-interesting answer boils down to materials science: it's all to do with how the atoms inside are bonded together. That explains how most materials work

In metals, for example, atoms are relatively weakly bonded (which is why most metals are fairly soft); their electrons are shared between them in a kind of sea that can "wash" right through them, which is (simplistically speaking) why they conduct electricity and heat. A material like rubber, on the other hand, is made of long-chain molecules (polymers) that are very weakly attached to one another; that's why raw, white, latex rubber is so stretchy and why black, vulcanized rubber (like that used in car tires) is harder and stronger, because heat-and-sulfur treatment makes strong cross-links form between the polymer chains, holding them tightly together. All the electrons are locked up in bonds of various kinds (none are free to carry an electric current), and that's why rubber is generally a good insulator.

Ceramics are different again. Their atoms are ionically bonded (like sodium and chlorine in sodium chloride, common salt), which holds them firmly in place (making ceramics hard and strong) and locks up all their electrons (so, unlike in metals, there are no free electrons to carry heat or electricity). Metals can bend, stretch, and be drawn into wires because their rows of regularly packed atoms will slide past one another. But in a ceramic, there are no rows of atoms; the atoms are either locked in a regularly repeating three-dimensional crystal or randomly arranged to make what's called an amorphous solid (a solid without a neat and tidy, internal crystalline structure). If you whack a lump of metal with a hammer, the mechanical energy you supply is dissipated as layers of atoms jump past one another; in other words, the metal bends out of shape. If you whack a ceramic such as glass, there's nowhere for that energy to go—no way for the glass to deform and soak up the blow—so it shatters instead. This explains why ceramics are both hard and brittle.