The art, science, or profession of teaching.

Since in Greek *agogos* means “leader,” a *paidagogos* was a slave who led boys to school and back, but also taught them manners and tutored them after school. In time, *pedagogue* came to mean simply “teacher” ; today the word has an old-fashioned ring to it, so it often means a stuffy, boring teacher. The word *pedagogy*, though, is still widely used, and often means simply “teaching.” And *pedagogic* training is what everyone majoring in education receives.

(1) A formal, unimaginative teacher. (2) A person who shows off his or her learning.

It isn't always easy to tell a *pedantic* teacher from one who is simply thorough. Some professors get an undeserved reputation for *pedantry* from students who just don't like the subject much. Regardless of that, a pedant need not be a teacher; anyone who goes around displaying his or her knowledge in a boring way can qualify.

A doctor who specializes in the diseases, development, and care of children.

Since *iatros* means “physician” in Greek (see IATR), words such as *pediatric* naturally refer to “children's medicine.” *Pediatrics* is a fairly new medical specialty; until about 1900, children were considered small adults and given the same medical treatment, only milder. Benjamin Spock was the most famous pediatrician of the 20th century, and his book *Baby and Child Care* changed the way millions of Americans raised their children.

(1) Of or relating to an encyclopedia. (2) Covering a wide range of subjects.

In Greek, *paidaea* meant not simply “child-rearing” but also “education,” and *kyklios* meant “general”; thus, an encyclopedia is a work broad enough to provide a kind of general education. The world's most eminent general encyclopedia, the *Encyclopaedia Britannica,* is a huge work that covers every field of human knowledge. But *encyclopedic* doesn't have to refer to books; it's often used to describe the wide-ranging knowledge that certain types of minds just can't stop acquiring.

In *hydrotropism*, a plant's roots grow in the direction of increasing moisture, hoping to obtain water. In *phototropism*, a plant (or fungus) moves toward light, usually the sun—perhaps because, in the colder climates where such plants are usually found, concentrating the sun's warmth within the sun- seeking flower can create a warm and inviting environment for the insects that fertilize it. In *thigmotropism*, the organism moves in response to being touched; most climbing plants, for example, put out tiny tendrils that feel around for something solid and then attach themselves or curl around it. When microbiologists talk about tropism, however, they're often referring instead to the way a virus will seek out a particular type of cell to infect. And when intellectuals use the word, they usually mean a tendency shown by a person or group which they themselves might not even be aware of.

(1) The decomposition of the matter and energy in the universe to an ultimate state of inactive uniformity. (2) Chaos, randomness.

With its Greek prefix *en-*, meaning “within,” and the *trop-* root here meaning “change,” *entropy* basically means “change within (a closed system).” The closed system we usually think of when speaking of entropy (especially if we're not physicists) is the entire universe. But entropy applies to closed systems of any size. Entropy is seen when the ice in a glass of water in a warm room melts—that is, as the temperature of everything in the room evens out. In a slightly different type of entropy, a drop of food coloring in that glass of water soon spreads out evenly. However, when a nonphysicist uses the word, he or she is usually trying to describe a large-scale collapse.

Any of a genus of herbs or shrubs having small white or purple flowers.

Helios was the god of the sun in Greek mythology, and *helio-* came to appear in a number of sun-related English words. The genus known as the heliotropes consists of about 250 species; many are thought of as weeds, but the best-known species, garden heliotrope, is a popular and fragrant perennial that resembles the forget-me-not. The heliotrope tends to follow the sun— that is, turn its blossoms toward the sun as it travels from East to West every day. But the fact is, *heliotropism*—turning toward the sun—is common among flowers (and even leaves), and some, like the sunflower, are more dramatically *heliotropic* than the heliotrope. Those in the far North actually use their petals to reflect the sun's heat onto the flower's central ovary during the short growing season.

Acting on the mind.

*Psychotropic* is used almost always to describe substances that we consume. Such substances are more numerous than you might think, and some have been known for thousands of years. Native American religions, for example, have used psychotropic substances derived from certain cactuses and mushrooms for centuries. Caffeine and nicotine can be called psychotropic. Psychotropic prescription drugs include antidepressants (such as Prozac) and tranquilizers (such as Valium). Any medication that blocks pain, from aspirin to the anesthetics used during surgery, can be considered a psychotropic drug. Even children are now prescribed psychotropic drugs, often to treat attention deficit disorder. And all recreational drugs are psychotropic. *Psychoactive* is a common synonym of *psychotropic*.