Why is the cashew the only nut you cannot buy in its shell? asks Russell Judge, via email.

Cashews -- the nut that sounds like a sneeze -- are the oddballs of the nut world.

Think of nuts on a tree, and you might imagine round walnuts hanging on the branches of a black walnut tree, thudding to the ground in late summer or early fall. But cashews come in an elaborate disguise. A cashew wearing its shell looks exactly like a fat worm, wriggling out the bottom of a misshapen apple.

Evergreen cashew trees grow only in a tropical or subtropical climate. The cashews we eat come mainly from India, Vietnam, Brazil, and a number of countries in Africa, including Nigeria and Tanzania.

Cashew trees can grow to be over 40 feet tall, their green leaves the backdrop for brightly colored cashew apples, the tree's "false" fruit. Yellow or red cashew "apples" actually look like pears, or oversized hot peppers. The nuts protruding from the apple's undersides are the tree's real fruit. Hidden inside each nut is a single seed -- a delicious cashew.

Why not pluck the nut and sell it with shell intact -- like a walnut, pecan, or almond? The problem lies in the cashew's family tree. One of the cashew's close relatives is the pistachio, the tasty green (though often dyed red) nut used for snacks and ice cream. Another is the tropical mango. But other relatives – the black sheep of the Anacardiaceae clan – include the not-so-nice poison sumac and poison ivy.

All of these plants contain urushiols, the oily chemicals that makes a brush with poison ivy such a painfully itchy experience. The cashew's share of urushiols are concentrated in an oily liquid trapped between the two layers of the shell. (It's no wonder that an old name for the cashew was "blister nut.")

Because of the lurking urushiols, cashews must be processed very carefully. Much of the work is still done by hand, and cashew workers often suffer from burning rashes and eye irritation. The process of removing the shells and extracting the liquid includes roasting, burning, boiling, soaking, cracking and peeling. Instead of being discarded, the cashew nutshell liquid (CNSL) is often sold for industrial uses. CNSL oils are used in waterproof paints, varnishes, and lacquers, while CNSL solids are used as friction particles in brake linings.

Finally, the cashew seeds are thoroughly cleaned and roasted. This leaves a batch of pristine (and urushiol-free) cashews, ready to eat.

Despite having poison ivy for a ne'er-do-well first cousin, cashews cause fewer allergic reactions than other nuts. But when cashew processing gets careless, there can be problems. In 1982, more than 50 Pennsylvanians who ate cashews sold by their local Little League ended up with poison-ivy-like rashes. The culprit: bits of cashew shells mixed in with the cashew pieces.

Fortunately, such glitches are rare. Cashews make an excellent snack, containing heart-healthy unsaturated oils as well as a good mix of protein and carbs. To see cashew apples and nuts on the tree, and learn more about their processing, visit this website.