Why do metals have grain?

Adding to Charles good explanation.....Did you ever notice water freezing on the windshield of your car? Much like metal solidification, crystals are nucleated at several or many points... and as the water freezes, the crystals grow outward.. read more

The grain size in a metal is a function of the proximity of the nucleation sites for crystal formation and growth. But with metals, most will recrystallize in the solid state when heated after the metal has been cold worked (hammered, bent, rolled, stretched). read more

If you have a pure piece of metal, you can control the size of the grains by heat treatment or by working the metal. Heating a metal tends to shake the atoms into a more regular arrangement - decreasing the number of grain boundaries, and so making the metal softer. read more