Rennet is Magic.

Rennet.  The magical substance that turns milk into cheese.  Alright so it's not actually magic.  That doesn't make the results any less fascinating. Rennet is an enzyme found in the 4th stomach of young ruminate animals.  It aids them in digesting their mothers milk by coagulating it into a semi solid to keep it in the digestive track longer.  Thousands of years ago humans stumbled on this coagulative property as a means of helping to preserve highly perishable milk.
Traditionally rennet was extracted by slaughtering the calf/kid/lamb, extracting said stomach, drying it, cutting it into strips, and adding little stomach bits to milk so the rennet could do it's work. It can also be produced (for you vegetarians among us) via microbes or the cardoon thistle.  So, for those who are squeamish, the slaughtering of adorable baby farm animals is not always required.  Most cheeses consumed within the United States in the present day are made with microbial rennet.
Now I am certainly not a chemist nor a professional cheese maker.  But in order to bring the basic concepts of how cheese is made to the masses I have created this informative Diorama of Learning!
With illustrations* by my own artistic hand.
 














*Also meaning extreme hyperbole and not-art.  In essence this is how rennet turns milk into curd.  Which is then made into cheese (fresh cheeses are another matter...).  Just without the rainbows, sparkles, and birthday cake...