A blue iris simply contains very little melanin. Light entering it is scattered by the iris fibers, and mostly the short, blue wavelengths bounce back out, a principle close to the one that colors the sky. According to a 2008 study, blue eyes may trace back to a single mutation that arose 6,000 to 10,000 years ago.
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.