Imagine never having smelled anything.  Ever.  Never knowing the scent of freshly brewed coffee, pine trees in the snow, or that funny aroma at a Grateful Dead show. According to The Taste and Smell Clinic in Washington, D.C., about 400,000 Americans lack a sense of smell. Today at the Experimental Biology meeting in Washington, Dr. Robert Henkin reports he’s found a way to restore smell in 75 percent of his patients. The answer is in nasal mucus. Which has growth factors that cause stem cells to mature and create smell receptors. People who can’t smell also lack a normal number of these growth factors.  Henkin increased their number with so-called PDE inhibitors, the class of drugs that includes Viagra. Smelling for the first time, patients face a child-like challenge: they have to learn  the relationships between objects and scents. Because most receptors are for sweet things, patients smell these first. Nice, that they can start with honeysuckle, coconut, homemade chocolate chip cookies. Instead of low tide.