Smell … a marvelous sense
Smell is a very direct sense. 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,
and a piece of fruit or whatever. Those molecules are generally 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.
When you smell many fruits or flowers, what you smell is esters evaporating from
the fruit or flower.