Between meals, roughly every 90 to 120 minutes, a powerful wave of contractions sweeps through your stomach and small intestine, pushing leftover food and bacteria further along. With little content to muffle the sound, the churned air and fluid echo more loudly. Those hungry rumbles are the sound of a deep clean.
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.