Dog has been a faithful friend of man for thousands of years. Most dogs are kept as pets to do useful work like herding sheep or guarding buildings. Today, there are more than 100 breeds of dog of many colours, shapes and sizes. Since they are short-sighted and see only shades of grey, dogs see a world that is blurred and has no colour. But, the sense of smell of a dog is thousand times stronger than that of ours. If it passes through one place, it can identify the same place again due to its acute smelling power. Therefore, trained dogs are used in the modern security system. They can sniff illegal drugs and culprits. Customs officers use specially trained dogs for sniffing out illegal drugs. It is not necessary to open cases or crates as one sniff is enough for a dog to find the drugs, even if they are packed in tins. In our nasal cavity, there is a yellowish area of about 250 square millimetres. This contains millions of hair-like cells, which are sensitive to smell. These are called ‘chemoreceptors’. They are always wet because of the mucus present there. Chemoreceptors are connected to the olfactory bulb in the brain. When we smell something, its particles along with the air reach the chemoreceptors. There they produce electrical impulses in the nerves. These electrical impulses reach the olfactory bulb and we identify the smell. The smelling power, in fact, mainly depends upon the size of the olfactory bulb. Bigger the olfactory bulb better is the smelling power. Studies have revealed that the olfactory bulb of a dog is bigger than that of a man, which is why the dogs have a greater power of smell. Another reason for this characteristic is that the nasal cavity of a dog is wetter as compared to that of human beings; this wetness further helps in identifying the smells. Dogs make use of the strong sense of smell in feeding, hunting and identifying enemies.