Hide your real hand, place a fake one in front of you, and have someone stroke both at the same time with a brush. After a minute or two, many people feel the touch on the fake hand, and flinch if it's threatened. First described in 1998, this illusion shows that the sense of “my body” is constructed.
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.