Why do you fire clay?

If clay is not fired then it can soften again and fall apart especially if put to use with moisture. I used to tell my students that firing was like a volcano...it was turning mud into rock with lots of heat and that was just a visual way to explain the change in chemical nature. Firing clay hot enough makes it vitreous, or glass-like. There are minute particles of earth that is "sand like" inside the clay formula that melt with heat and bind the particles of clay together so they cannot be turned back into water and mud! It's important to know the composition of the clay to know the proper temperature to mature it.
Primitive firing was done in an earthen pit with a huge pile of wood or other burning matter set on fire and burned for several hours until the pots are just covered with a bed of very hot coals.