The ability of a material to flow is called its ductility. Glass and some other materials are not very ductile because the bonds between the atoms making up the material are extremely strong.

Glass is a sub-crystalline solid, made of the same stuff as quartz. Crystals are usually very brittle, because in order for the atoms in a crystal to be arranged in such an orderly way to make a crystal, the crystal itself must be very, very rigid. Rigidity makes for hardness, but also brittleness. Compare this to protein (which your skin is made out of), which is much, much softer, but much less rigid and thus less brittle.