The visual cortex doesn't sit idle when the eyes send nothing. In people blind from birth, it lights up as their fingers run across Braille. Briefly disrupt that area with magnetic stimulation, and their tactile reading falters. The brain repurposes its territory for whichever senses need it most.
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.