Normally your lungs' air passages are lined with millions of tiny hairs called cilia. The cilia act like little brooms protecting the air tubes by sweeping dusts,tar,and other foreign materials gradually upward,like escalators,until they can be spit out. Every time a blast of tobacco smoke hits these cilia,however,they slow down,and soon stop moving. As a result,the trapped tars from the tobacco smoke begin boring into the cells lining the air tubes. Over time,this constant irritation turns some of the cells cancerous. This transformation takes many years. But once it begins,the cancer steadly eats its way deeper into the lung. By the time it is discovered,it's usually too late.