When you think about crystal, do you imagine a geode, glistening at its opening or maybe imagine the the rough shafts of gypsum and feldspar that stick out at every angle from cave walls? Maybe the geeky out there will remember LCD monitor is a liquid crystal display, so surely it must have crystals in it. Whatever the image, with their sparkling beauty and rarity, crystals have taken on a captivating aura. Some have even taken crystals to hold healing powers.
But nearly everything solid in the world is made up of crystals! Not even rare, they're far cheaper than a mere dime a dozen. Even that dime is made of crystals! The dull granite that make up the ground, the metals that make up a car, and the ceramic that makes up your coffee cup all have a crystal structure. This is because of the somewhat-surprising fact that atoms bound together in a crystal require less energy than those same atoms spread out independently. As things tend to settle into the state that requires the least energy, things cooling down from a liquid into a solid tend to form crystals. And, viewed on a cosmic time scale, nearly everything on the Earth was molten at some point.
Consider salt, or, as the geeks like to call it, halite. Salt is also a crystal. If you pick up a little grain of salt and look at it, it sparkles like you'd expect of a crystal. But even a 1mm little grain of salt is made up of several even smaller crystals, meeting at weird angles. Each of those little crystals is called a 'grain' and where they meet are called 'grain boundaries'. Under a microscope, these boundaries spread all over like a web of cracks.
If you push on an aluminum bar, it will start to deform a little before it obviously crumples. This is accomplished by the individual grains slipping along the grain boundaries. It takes much less energy for grains to rearrange themselves than for a crystal to break apart. So rather than break, the grains slide around until the defects in the crystals collect together and the grain boundaries start pressing against each other. At a certain point, nothing more can slide and these little cracks join together, forming big cracks. Finally, they grow large enough to be seen by the naked eye, and things begin to fall apart.
So it is that most solid things — almost everything of consequence in our daily lives — are composed of a whole bunch of little bits that exist between the cracks.
For a robot.