Touch your thumb to your little finger and flex your wrist slightly: if a tendon pops up in the middle, that's the palmaris longus. About one person in five lacks this muscle on one or both sides, with big differences between populations, and with no noticeable loss of strength. More useful in other primates, it's often cited as an evolutionary leftover.
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.