Brown fat, packed with mitochondria, doesn't store energy: it burns it to make heat. It was long thought to exist only in babies. In 2009, several studies showed that many adults still have some, especially around the neck and collarbones, and that it switches on in the cold. A discreet internal heater.
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.