A stroke occurs due to a sudden stop of blood flow to a part of the brain. It can happen either due to, 1. A blood clot in the supply route: 80 per cent of strokes in the US occur like this. 2. A bleeding in a supply route; happens due to damage to an artery wall. The above image is of a stroke due to a blood clot: (Image source: https:/ / commons.wikimedia.org/ wiki/ File:Stroke_ischemic.jpg. This image is in the public domain of the NIH US website.)