The kidneys 'clean' your blood by filtering out excess water and waste, which then becomes urine. During this process, minerals in the urine may crystallize inside the kidney, and form tiny stones. Some get no bigger than a grain of sand, while others may grow to golf-ball size. The smallest stones are carried out through a tube called the ureter (YOUR-ih-ter), into the bladder, then pass unnoticed during urination. However, those just large enough to block the ureter can cause excruciating pain. During an attack, urine may be bloody, and you may experience fever, chills, nausea, or vomiting. This condition requires prompt medical attention, even if symptoms go away. Eventually, stones less than one-quarter inch in diameter usually pass on their own, though painkillers may be needed until this happens. To help flush the stones out, you'll be advised to drink two or three quarts of water a day. Larger stones may require some intervention. High-energy shock waves, laser, or air pressure can be used to break the stone into smaller pieces; a catheter may be inserted into the ureter to remove the stone; or the stone may be accessed through an incision in your back. Surgery which actually opens the kidney is performed only as a last resort. You can reduce your chance of kidney stones by drinking lots of water daily. For more information, consult a specialist.