Always wondered about this one, but until Rebecca Mocaby of Southeast called, I never knew the answer. Now I do, and I hope it'll spur some Washingtonians to action.

Rebecca's question was whether letters addressed to Santa Claus are kept by local post offices or junked. I told her I suspected the latter, since post offices have better things to do at this time of year than worry about fantasies, and since it's doubtful that anybody named Saint Nicholas Claus will drop by to ask for his mail.

But I was wrong, not for the first time. Local post offices do indeed keep letters addressed to Santa -- and in many cities, citizens are invited to drop by the post office to answer them.

At the main P.O. here, individuals and groups are welcome to come down to reply to Santa letters, according to George Conrad, a public affairs officer.

"First we separate them according to whether they have return addresses or not," said George. "Then we make them available for people to take a look at. They can select what they'd like to answer."

Usually it's individuals who do the answering, George says. But sometimes groups like the Salvation Army drop by.

"We suggest people call first," George said, "especially since they're usually looking for letters from needy children." However, those can be hard to identify, George points out, unless a child mentions that his parents are out of work.

Santa answering hours are 8 a.m. to 5 p.m. weekdays, at the main P.O., North Capitol Street and Massachusetts Avenue. The phone number is 523-2001. If you have a few hours to spare, or if you know of a few people who might, I can't think of a better way to spend a December afternoon.

On the other hand, you may feel that Christmas shopping is the best way to spend a December afternoon. But you may soon discover otherwise if you don't anticipate a few pitfalls.

Luckily, the Better Business Bureau of Washington, D.C., has anticipated most of the real baddies for you. Here's the BBB's list of what to watch out for in yuletide consumerland:

*Be particularly careful about knowing the refund and exchange policies of stores. Be sure you're clear about when the store makes cash refunds, when it will file a credit memo with your credit card account and when it will issue a store credit.

*Be sure store receipts show the date and location of purchase. Without both, you have little hope of getting a refund, or a replacement for a defective item.

*Make sure that the store policies concerning items marked "sale" or "clearance" are the same as the policies for items carrying the regular price.

*By law, consumers are allowed to read the warranty on any household appliance costing $15. If it doesn't have a warranty, or if the store won't show it to you, and you buy the item anyway, you're taking a big, needless risk.

Wisdom from Vance Garnett of Silver Spring:

"Observed at the corner of 15th and K Streets NW -- a gold Mercedes with a clashing orange boot.

"How can one afford a Mercedes but not afford to pay three parking tickets?"

From "a federal bureaucrat who would love to give you his name but who works for the subject of the joke:"

It has been more than a year now since Ronald Reagan said, "You ain't seen nothing yet."

By gosh, he was right.

Seasonal witticism from Damian McCoy of Northeast:

A holiday is a day off. It's usually followed by an off day.

From Mitchell H. Allen of Fredericksburg:

"I like to doodle with letters and numbers. One type of doodling involves transforming a string of letters into its numerical equivalent, and vice versa.

"Can you imagine my shock at discovering that the alphabetic equivalent of 495 is DIE?

"Perhaps this explains why that section of the Beltway is so dangerous."

*CHILDREN'S HOSPITAL

Many are the reasons that accompany gifts to our annual fund-raising campaign on behalf of Children's. Here are three that touched me -- and that might inspire you.

The first is from Lynn Calvetto, a student at Mount St. Mary's College in Emmitsburg, Md. No creature on earth endures a less bountiful budget than a college student. But that fact didn't stop Lynn from sending along $5.

Thank you so much, Ms. Strapped Collegian. How about it, Lynn's fellow scholastic sufferers? Think of a $5 contribution as one pizza uneaten, or one movie unattended. Is that so terrible a price to pay when the money will help a sick kid get better?

The second letter comes from Cathy and Tom Ager of Falls Church. Their daughter Elizabeth was 2 months old on Dec. 2. In her honor, Cathy and Tom contributed $25 to the 1985-86 campaign.

"We now realize how precious every child is to its parents," the Agers write. They had delayed giving to the hospital drive in the past. Now, as new parents, they vow to do so no more.

If you're a parent like the Agers, you know that your child's welfare is never far from your mind. But some kids get seriously ill, and their families can't afford the cost of medical care. Helping such families is what our campaign is all about. The Agers have done their part. Won't other parents do theirs?

The third letter is from Eva Popovich of Arlington, who proves that she understands the concept of cash flow very well, indeed.

Eva mailed in a $30 check with a note that said, "I know that if I did not sit right down and write you the check for Children's Hospital, it may not get written until after the Christmas rush."

I salute your promptness, kind lady. Of course, you could have waited until after the Christmas rush. The New Year's rush, too. The campaign does run until Jan. 24, after all.

However, every dollar I can ship on to the hospital early in the campaign helps that many more kids sooner. To delay is to risk forgetting. Won't you find a blank check right now and do what Eva Popovich did? You'll feel better. So will a child.

Memo to those of you who inhabit offices:

One week from today, life as you know it will cease for the rest of the year. Many of you, and many of your coworkers, will head elsewhere for the holidays. That will greatly shrink the line at the office coffee machine. But it will also greatly shrink the pool of protoplasm that can be hit up for a contribution to Children's.

Offices are wonderful places to raise funds for the hospital because it's so hard to say no to someone you work with. On the street corner, you might well keep on walking when "the touch" comes. But how are you going to say no to Good Old Joe when you know darn well that you're going to borrow his scissors next week?

In every office, a Good Old Joe lurks. Appoint yourself he. In the week before holiday desertion strikes, pass the hat for sick kids among all the scissors-borrowers you know and love so well. The kids are counting on you. So am I. TO CONTRIBUTE TO THE CAMPAIGN:

Make a check or money order payable to Children's Hospital and mail it to Bob Levey, The Washington Post, Washington, D.C., 20071.