The “crack” comes from a gas bubble that suddenly forms in the joint fluid as it's stretched, as captured on MRI in 2015. Several studies have found no link with arthritis. One doctor even cracked the knuckles of only one hand for 60 years: no difference between his two hands.
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.