If you want to search out the best way to make the most of a relationship, you will want to use plain language, perhaps with some padding out.

If you want to explore the best way to exploit a relationship, you will want to use explicit language, perhaps with some expletives.

Somehow the second sentence has a very different tone from the first. And yet they can mean the same thing. It's just that somehow the expl words have acquired a — shall we say — shady tone to them. How? Allow me to explain.

There are just seven basic words in common use in English (plus words derived from them) that start with expl, and they're not all related. What's more, none of them originally have to do with anything sexual or negative. And none of them now mean only something negative or sexual. But take a look at this bunch, some of which, like a movie group of high school friends, turned out to be bad apples:

Explain: Latin source: explanare, from ex "out" and planus "flat." Meaning: unfold, spread out, make plain — now figuratively. It may not seem like an unpleasant word until someone says, "Let me explain something to you." But it has no sexual overtones. Yet.

Expletive: Latin source: expletivus, from ex and plere "fill." Meaning: serving to fill out or pad out. Linguists use this word for such things as the empty "it" in "It's raining." Everyone else uses it to mean "vulgar word."

Explicate: Latin root: explicare, from ex and plicare "fold." Meaning: unfold, unroll, expand — now figuratively, mostly as a ten-dollar word for "explain." It has managed to escape negative connotations...for now.

Explicit: Latin source: explicitus, from ex and plicare, same as explicate. Meaning: open, plain, straightforward, undisguised, not implied. Now typically implying frank sexual displays or references.

Explode: Latin source: explodere, from ex and plaudere "clap." Meaning: this word originally meant to drive a performer off the stage by clapping and hissing. Really! From the "drive out violently and noisily" sense it ended up with what happens when gunpowder or high-pressure gas causes a sudden rupture and release. Not a pleasant word, but not a naughty one. So far.

Exploit: Latin source: explicitare, from explicare (again!). Meaning: make good use of, or make use of for personal gain. Somehow we can talk about heroic exploits, but exploitation is plain old bad, and often sexual too.

Explore: Latin source: explorare, from ex and either plorare "shout" or pluere "flow." Meaning: search out, investigate. Not an intrinsically sexual word, but it can easily become one.

From their innocent origins, three of these — expletive, explicit, and exploit — went over to the darker side, one after another, and may be going to lead the others astray too. It's time to expose what happened.

Exploit was the first of them to start turning rotten. Back in the 1830s it was already being used to mean making use of something for selfish ends. But something else happened in the 1960s: "exploitation films" started coming out, cheap movies aiming to exploit a current hot topic loaded with lurid material for commercial gain. The echo of sex in the ex helped this — sex-exploitation film readily became sexploitation film, and the overtone stuck. Exploit was no longer simply greedy. It was now decidedly seamy. The term sexual exploitation first became common in the 1970s.

Those exploitation films, along with other naughty movies and magazines, came to be of increasing concern to lawmakers and other upstanding citizens. They wanted a technical-sounding word for what they were talking about. They found this word explicit, which didn't at the time have specific sexual overtones — and just happened to sound a bit like exploit too. So the phrase sexually explicit came into regular use in the early 1970s, and so did use of explicit by itself to mean "sexually explicit."

Something else happened in the early 1970s: Watergate. Richard Nixon had been taping conversations in the Oval Office, and when the investigation into the Watergate break-ins started, they had to deal with tape transcripts that were full of naughty words. What euphemism did they use for these rude filler words? Expletive — as in expletive deleted. All of a sudden an odd little linguistic term was pressed into service as a synonym for "vulgarity." Why that word? At the time it seemed so technical — though it did just happen to have that expl, as in explicit, exploit, and maybe explosive, which is what the tapes certainly were.

An added bump to explicit came in 1985, when Parental Advisory labels started being put on music in the U.S. The first songs to get the label were ones such as Prince's "Darling Nikki," which doesn't have any profanities but is rather frankly naughty. The labels have become common, and now a song that has nothing to do with sex but is padded out with vulgar words will also get a warning of "explicit" language — by which is just meant expletives, in the new sense.

So what we had was a perfect storm of conditions to get those three similar-sounding words feeding off each other. Are they starting to have a significant effect on other expl words — such as explore? Time will tell. The effect of sound resemblance may yet explode.

But does sound resemblance really have an effect on what we expect a word to mean? Let's try out a word that hasn't been in use in English for a few hundred years: explement.

What does explement make you think of? What does it seem like it should mean? Is it a word you would feel comfortable saying to your mother?

You should recognize — when you look at it — the relation to complement. An explement is just something that fills up, a fulfillment. But I'm betting that you can't quite get over the expl — and the resemblance to excrement as well, even though it's not related. If we start using explement again, I suspect it will be used to mean not just any fulfillment but a particularly unpleasant one.

And if other expl words get increasingly sleazy overtones, it will just be the explement of the bad-apple theory of sound resemblance.