To be able to smell something, molecules from what you're smelling must be able to float through the air and up your nose to neurons that perceive smells. The molecules need to be light and volatile (easy to evaporate) so they'll float. Substances with these kinds of molecules - such as some flowers and many foods - are easy to smell. At least they're easy to smell if you have the gene (and if the gene in your DNA is undamaged) for encoding your receptor neurons to recognize that particular smell.
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