Singled Out: "Being single is not a phase like the 'terrible twos' or adolescence," declares Joan H. Lewis of Bethesda in response to Therese Keane's more ambivalent view of singledom.

"Being single is itself a life style," writes Lewis, who is a free-lance writer, editor and public-relations consultant. "And like other life styles (marriage being the most common), you have to work at it if it is going to be successful."

Other exhortations in Lewis' soliloquy for singles:

"Don't put off something you want (and can afford) 'until you get married.' Taken to extremes, this philosophy could mean that you eat off paper plates for the rest of your life. Instead, build a style for yourself. Be aggressive about what you want from life. Be a little selfish; it's good for the soul."

She suggests passing this list on to your married or living-together friends:

* Invite me to parties of couples if I am friendly with more than just the two of you. I enjoy seeing people I like.

* Plan small parties with interesting guests of whatever gender or marital status. I don't always need to be "matched up."

* Don't be afraid to be frank about marriage for me. Sometimes I want it; sometimes I don't. If we're really friends, I'll let you know what's on my mind today.

* Invite me to your child's birthday party, but don't be offended if I decide I can't take the decibel level. I like the silence of my apartment sometimes.

* Offer me use of your country home or beach condo, but don't feel sorry for me and offer it rent-free.

* Introduce me to interesting people, even if they are recently divorced or separated. If we're interested in each other, we'll work it out. If we're not interested, we'll work that out, too.

* Let me share my concerns about my life style with you, as I hope you will do about yours with me.

* Understand that single is not always a temporary life style. Not all single people are on hold.

* Wait a Second: If you've wished for more time, you're about to have it. Start planning now for a big June 30. The last minute of the last day in June will have 61 seconds rather than that paltry 60.

To clock the details: A "leap second" will be inserted at 23:59 Greenwich time (7:59 p.m. EDT) in an effort to keep atomic time closely matched to solar time. The folks at the U.S. Naval Observatory assure us that this decision is an "international scientific agreement," and that you had the same chance June 30 last year. You could get that extra second next year, but you may have to build your plans around its occurrence in December.

You're sure the world is moving faster? You're wrong. It's slowing down. We have about two hours more each day than the dinosaurs had 150 million years ago. And consider the poor critters 300 million years ago who had to make do with only about 20 hours.

* On the Track: Ruth Klimpl, Alexandria program coordinator/infant development specialist, twits us gently for omitting the Alexandria Parent-Infant Education (PIE) program in a list of resources for tracking early childhood development.

The Alexandria PIE program provides free developmental screening and consultation for infants up to 30 months old whose parents are concerned about possible delays in development or the effects of prematurity or complications at birth. Evaluations are performed in the home, and an individualized educational program is provided if needed. For more information or to set up an appointment, parents may call 838-4458.

"The growing interest in early intervention, and especially the support of local pediatricians and hospitals, is encouraging," writes Klimpl, "to those of us who are aware of the value of specialized help as early as possible."

* Teen Typing: Lisa Goldberg, 15, who will be a junior next year at Springbrook High School, Silver Spring, sends in this lament:

"In these progressive times, this may sound strange, but I must confess that I've never had sex, taken drugs, been in a car crash, shot my parents, stayed out all night, overdosed or run away from home. I am a member of that large but greatly ignored group--the straight teen-agers.

"Contrary to what the public seems to think, many teens are perceptive, intelligent and very aware of all the advertising hype that is projected into their image: A 'typical teen-age girl' is a bubbly blond bimbo pemanently glued into her cheerleader's uniform. Her motto is, 'Never go through the entire day without squealing at least once.' The typical teen-age boy is the same, with a few exceptions--he wears a football uniform and grunts instead of squeals.

"All this wouldn't be so harmful if it weren't for the fact that many teens are beginning to believe the image that the media projects. 'Perhaps there's something wrong with me,' thinks the boy who, after hearing morbid statistics on the news, wonders if it's abnormal not to go out drinking with the guys every Friday night.

"It's even worse for the girl, who has a double dilemma: Either she dreams of outranking Marsha Brady or she feels miserable because she hasn't had sex by age 16.

" 'Don't forget your hair, face and figure!' scream the advertisements. 'Get a soft perm, spray on instant blond hair, brush your teeth for fresher breath, become gorgeous in 12 seconds, eliminate static cling and build up your muscles so you can look like Superman in a year!' Actually I don't think too many people are worried about their static cling . . .

"Instead of advocating sex, drugs and booze, why can't the media try a different campaign that has nothing to do with growing up too fast? Adults aren't the only ones who feel as though their intelligence has been insulted when ads insist that the only thing a head should hold is a dye job . . ."

"There's nothing wrong with growing up fast," muses Lisa, "unless you're not ready for it."

When she is, "I'm going to be a famous writer. Maybe I'll write a novel about being a teen-ager."

* Whale of an Idea: Everybody, it's no secret, needs somebody to love, but 40 tons off Cape Cod? For all of you out there whose maternal and paternal instincts surfaced over Carol Krucoff's story on adopting a humpback whale, a blatantly blubbery apology.

In our eagerness to tell you that you can have a whale to call your own, we neglected to include the address where you can get names and a rundown on the charms and idiosyncracies of 70 of the endangered species. For $10 or $15, you'll get an official adoption certificate, your baby's picture and the Whalewatch newsletter. Write: Whale Adoption Project, c/o International Fund for Animal Welfare, P.O. Box 193, Yarmouth Port, Mass. 02675.