The 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.
As we’ve already seen, not all ceramics behave this way. Graphite is soft because it’s made of layers of carbon atoms that will slide and shear (that’s why a graphite pencil leaves lines on paper); diamond is hard because it has a much more rigid crystalline structure. Clay dug from the ground is soft and pliable because, like graphite, its atoms are made of flat sheets that can slip past one another, held together only by weak bonds. When you add water to clay, the polar water molecules (positively charged at one end, negative at the other end) help to pull those bonds apart, making the clay even more malleable. When you fire clay, the water evaporates and the aluminum, silicon, and oxygen atoms lock into a rigid structure made from aluminum silicate, bonded together by silicate glass—and that’s why fired clay is so hard.
Artwork: Why do ceramics and metals behave differently? 1) You can bend metals because the atoms inside them can slide past one another fairly easily. 2) In a ceramic, the atoms are tightly bonded. If you apply too much force, the only thing a ceramic can do is break apart: the energy has nowhere else to go. 3) In metals, there are free electrons (blue) to carry heat and electricity. That’s why metals are good conductors. 4) In a ceramic, the electrons are all “busy” binding atoms together and there are none spare for carrying electricity and heat. That’s why ceramics tend to be good insulators (non-conductors).