Milk is full of a sugar called lactose, which needs a special enzyme called lactase to break it down. But children used to stop producing the enzyme around the age of 4 or 5—and those of us who still make it are, in fact, mutants.
When I was a kid, there used to be a mason jar in my house that was always kept just out of my reach. It looked like it was filled yogurt, but grosser. Runny, chunky, with a sickly yellow color. I didn't find out until years later that it was actually the most delicious milk liqueur I've ever tasted.