We all know that smoking is harmful to our health. Smoking is very dangerous. This is because cigarette's smoke contains at least two harmful substances － tar and nicotine. Tar, which forms as the tobacco, burns and damages the lung ，and therefore affects breathing. Nicotine, which can be found in the leaves, causes the heart to beat faster and increases breathing rate. When we know this, we should stop smoking. For your health as well as others, please don’t smoke or give up smoking.