Questions for the Age of Mobility:

* When the mother in Bowie has custody on weekdays and the father in Bethesda has custody on weekends, where do you send their daughter's birthday present?

* How can you be sure the loving couple to whom you addressed an anniversary card last year is still together and celebrating?

* How do you know whether the mother of your son's new playmate uses her maiden name or her married name -- and whether it is still the same married name she had when her son was born.

These days, people are changing partners faster than square dancers. Even in a transient town, this is ridiculous!

What we need is a guidebook: a constantly updated Who's Who that will tell us what's what with the people around us. The authors can skip the professional accomplishments and the academic degrees and simply tell us who's in the family -- and who's out.

Churches and schools could compile the information as a public service and enterprising high school students could earn college tuition by distributing it. It would help us avoid the constant embarrassment of asking the wrong questions and calling people by the wrong names.

Not long ago, I phoned to find out what time another parent was picking up my son for a hockey game at the Capital Centre. A man answered. I asked for the mother, Mrs. McAdams, knowing that the child's last name was McAdams. The man answering the phone was instantly irate.

"She's Mrs. Finch and don't you forget it!" he shouted.

After I swore a blood oath to call his wife Mrs. Finch forevermore, Mr. Finch calmed down. "I thought you were the piano teacher who refuses to acknowledge our marriage," he explained.

Now, why couldn't the class list have explained that Barney McAdams' mother is really Mrs. Finch?

The old marriages are no easier to keep track of than the new ones. I recently bumped into a swim club acquaintance I hadn't seen for several months.

"How's your husband, Donny?" I asked casually.

"I believe he is selling commerical real estate. I haven't seen him for months," was the frosty reply.

Let's hope that before the next season the swim club will send out a new membership roster noting Donny's change in marital status.

Tiptoeing through the tulips of separation and divorce is a necessary social skill and considerate hostesses should consider the current climate when they send out invitations to parties.

Along with the driving directions should come the social directions: who's who and what's changed since the last February Frolic or Bastille Day celebration. The information could take the form of those chatty Christmas letters some people still send:

"Remember Fred and Martha? Well, it's Fred and Marilyn now. Martha has moved to Milwaukee with her new husband George. Alison will be coming alone. Don't even mention David. Warren also will be alone -- but you may ask about Eloise, she's only away on a business trip. Just don't ask if he's learned to cook. And the young man with Deborah is not her son. He is her son's ex-roommate who has just become her roommate. Deborah requests that you keep your opinions to yourself."