I was flipping through On Food and Cooking by the awesome Harold McGee tonight and stumbled on the #1 fact of all time:

Cheeses made with rennet melt, while cheeses made with acids don't. 

This is amazing! We've all tried to make a grilled cheese with some leftover queso blanco (haven't we?) only to discover that it doesn't work at all, much to our disappointment.  Well, now you know that it's because queso blanco, along with things like paneer and ricotta, is curdled only with an acid. 

And what's made with rennet? Mozarella, cheddar, provolone, the sorta stuff that you'd put on a grilled cheese if your fridge was stocked with more than queso blanco. 

So, what is rennet? A long, long time ago, rennet was made from the 4th stomach of a very, very young calf (less that 30 days old!), that was soaked in a brine that sucked the necessary enzymes out, but nowadays it's mostly made by processing frozen stomachs in a production setting. The stomach contains chymosin, an enzyme that allows milk proteins to bind together. 

And finally, what's the difference between acid cheeses and rennet cheeses? I can't explain any better than Harold, so here's a nice long quote. 

Rennet creates a malleable structure of large casein molecules held together by relatively few calcium atoms and hydrophobic bonds, so this structure is readily weakened by heat. Acid, on the other hand, dissolves the calcium glue that holds the casein proteins together in micelles and it eliminates each protein's negative electrical charge, which would otherwise cause the proteins to repel each other. The proteins are free to flock together and bond extensively into microscopic clumps.

That's science for you! 

Side note: Paneer is one of the easiest cheeses to make at home, and it's super satisfying because your homemade version totally resembles the stuff you get at restaurants. The only skill required is the ability to juice some sort of citrus and put it into a pot of milk.