Common causes of stroke come from blood vessels both outside and inside the brain. Atherosclerosis (hardening of the arteries) can occur when plaque (deposits of cholesterol, calcium, fat, and other substances) builds up and narrows the vessel, making it easy for clots to form and further occlude the vessel. The clots can break free only to travel to and block smaller vessels inside the brain. The blood vessels inside the brain itself can accumulate this plaque. Occasionally, weakened vessels can burst and bleed into the brain.