In most mammals, production of lactase, the enzyme that digests milk sugar, shuts down after weaning. That's still true for about two-thirds of humans. In the rest, mutations that arose less than 10,000 years ago, independently in several herding populations, keep lactase switched on for life.
Every day, your kidneys filter around 180 liters of blood plasma, about a bathtub's worth. Yet you only have about 3 liters of plasma, so it passes through your kidneys some sixty times a day. Over 99% of the filtered water is reabsorbed, leaving just one to two liters of urine.
A 70 kg adult is made of about 7 × 10²⁷ atoms, or seven billion billion billion. The observable universe is thought to hold on the order of 10²² to 10²⁴ stars. Even with the high estimate, you contain thousands of times more atoms than there are stars in the entire visible cosmos.