How do you crack corn?

Corn is a grain much like wheat or rice and you can make flour out of it (called corn meal) too. HOWEVER corn has a very hard kernel and if you were to put the whole grain into a grind-stone it would wear grooves into it. (Not a good thing to happen to a grind-stone). So before you can mill corn you have to crack the kernels open. This is done by another grind-stone, one that is not so expensive as the real one, not expected to do such a fine job, doesn't have to be perfectly smooth, if you know what I mean. I've seen people do this by hand using a grooved tray and a rolling pin type grind-stone.

Cracking and grinding can be done with the same stone, it is a matter of raising or lower the top stone to make the gap the corn in ground in narrower or wider, making the corn fine or coarse.