Dogs have extremely keen sense of smell. Not only do they have many more receptors for identification of smell in comparison with the human beings but their receptors are about 100 times longer. Hence, many times more molecules of smell can gather on them to boost the capability of identification by smell. The dogs belonging to the breeds such as spaniel, labrador, collie, and retriever etc. are naturally more skillful in searching and bringing back birds shot by the hunters which might have fallen in the jungle thickets. Therefore, the dogs of the above mentioned breeds are trained to sniff out the bombs. But more important is their training which takes about 12 weeks. The training of sniffer dogs has to be meticulously planned. The nature of smell to be identified must be made known to the dog during the training. Initially some specific type of explosive is wrapped in newspaper. Then the package is hidden in the training camp. The dog is expected to bring that package back to the trainer held gently in the mouth. Successful performance of this work brings suitable rewards for the dog. Even hugging and fondling by the handler or simply playing with it is also a reward. The dog must know that it has got a pleasant reward for sniffing out the parcel having a specific type of smell.
Another important thing: Before embarking on the training for identification of the explosives a special type of halter of leather straps is put around the dog’s neck and chest which is not done except when the explosives have to be sniffed. So the dog comes to know that it is being ordered to sniff out explosives. It must be impressed in the dog’s mind that what really matters is the smell and not the package that gives out the smell. Hence, the explosives are successively wrapped in different types of packages. The packaging may vary but not the explosive such as ammonium nitrate or RDX. Since the rewards follow the identification of smell of explosive the dog learns to concern itself with the smell only. During the course of intensive training the sniffer dog is trained to identify about 12 types of explosive materials.
Detection dog (Wikipedia)