Sorry I was slow to answer! In order for you to smell something, molecules from that thing have to make it to your nose. Everything you smell, therefore, is giving off molecules — whether it is bread in the bakery, onions, perfume, a piece of fruit or whatever. Those molecules are generally made of small and light, volatile (easy to evaporate) chemicals that float through the air into your nose. A piece of steel has no smell because nothing evaporates from it — steel is a non-volatile solid.
At the top of your nasal passages behind your nose, there is a patch of special neurons about the size of a postage stamp. These neurons are unique in that they are out in the open where they can come into contact with the air. They have hair-like projections called cilia that increase their surface area. An odor molecule binds to these cilia to trigger the neuron and cause you to perceive a smell. Impressive huh?