The thymus, tucked behind the breastbone, is a school for T cells: it's where they learn to spot intruders without attacking your own cells. Very active in children, it starts to shrink early, especially after puberty, and gradually fills with fat. But the “graduated” T cells keep protecting you.
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.