If you think about lactation too hard, it starts to seem a little strange — like the biological equivalent of saying the word "that" over and over until it's just a weird sound you're making. But, writes Nicholas Day at Slate, the sort of existential weirdness of breast milk is nothing compared to what's going on in the stuff at a chemical level. For instance, breast milk contains sugars that aren't actually digestible by human infants. That's because they aren't meant for the infant, itself. Rather, your breast milk is helpfully feeding your baby's intestinal bacteria. Freakier still: In monkeys, the chemical composition of breast milk can change, depending on factors like your baby's sex and whether your baby is showing signs of illness.