Dear Mary Jane and Uncle Sam,

Few frustrations equal a child's questions when they divert one's attention from important adult concerns. The particular concern which comes to mind is my annual attempt to spot a flicker of intelligibility within the prose of a Form 1040 instruction manual. The child who intruded is the son of a woman who cleaned my office.

She found me at my desk after hours, munching pencils. Explaining she could no longer afford a baby sitter, she asked would I mind if her boy sat quietly in a chair while she dusted, vacuumed and emptied wastebaskets? Not at all. Moments before their entry, I discovered a piece of depreciating property had acquired, thanks to an undefined something called Asset Depreciation Range System, a ''midpoint class life,'' and I would never notice the lad.

She and the vacuum wandered into a back room. I commenced searching for a definition of ''midpoint class life'' (or ADRS . . . or any other term in the manual). The mini-inquisitor slid from his chair, wandered up, stared at my papers and inquired, ''Do you know Mary Jane?''

''What?''

''Do you know Mary Jane?''

''Um . . . I was in high school with a Mary Jane . . . somebody.''

At the precise moment when I understood (I think) that Asset Depreciation Range System is not the same thing as Accelerated Cost Recovery System and began to realize I would be well beyond the midpoint of life before I knew what either meant, the interrogator whispered, ''Doe?''

''What?''

''Doe.''

''What are you talking -- as in deer? Or re, mi, fa?''

''Mary Jane Doe.''

''Oh, for goodness sake. Mary Jane Doe is a pseudonym . . . or an acronym, maybe . . . or -- well, whatever nym it is or isn't, Mary Jane Doe does not exist.''

The boy returned to his chair. He sat still for three minutes, pondering, while I shuffled assorted paraphernalia of adult concerns. I had just taken pencil to Form 1040 when he wandered up to the desk again. Quickly I covered my sailboat doodle in the Adjustments to Income column. ''She does, too,'' he stated.

''Who does, too, what?''

''Mary Jane Doe does exist.''

''What makes you think so?''

''I saw her address. 123 Main Street, Anywhere USA.''

Mentioning that Anywhere USA was as fictitious as Mary Jane Doe (and probably as illusory as an Accelerated Cost Recovery System) plunged me into a revival of the Nym situation. It was useless to protest, in any event, because the lad had proof in pocket. The manufacturer of a self-inking address stamper had filled an entire magazine advertisement page with a photograph showing his product imprinting Mary Jane's address in blue ink.

''Can't you tell I'm busy? Uncle Sam is waiting for his money.''

My exasperation led to an unexpected pitfall.

''Uncle Sam isn't real,'' the boy informed me.

What prompted me to tackle that matter escapes me, now, but I found myself trying to explain why, if Uncle Sam belongs to the same nonexistent Nym family as Mary Jane, Sam gets lots of money from everybody while Mary Jane cannot receive letters from anybody. Avoid this pitfall -- at whatever cost. Otherwise, you risk the embarrassment of hearing yourself vow, ''No, it doesn't sound fishy to me . . . what Sam gets, actually, is a tax -- TAX -- not a bribe . . . all right, Miss Doe is probably a very nice lady, but I am equally certain she prefers you to me in her quest for pen pals.'' Now, there was an idea! ''Why don't you write Mary Jane and ask her how things are in Anywhere?''

Some weeks after I mailed my tax return, I was late getting away from the office. The young correspondent accosted me with a shriek. ''She wrote!''

''WHAT?''

''Mary Jane answered my letter.''

He unfolded a sheet of paper for my inspection. In very neat handwriting, Mary Jane expressed her appreciation that a young admirer should notice her address in a magazine and trouble himself to write her. She was fine and hoped his nice friend, who growls at his calculator and taps a chewed pencil on his forehead, had finished his tax return. Finally, she apologized that this note must be her only correspondence; she was about to move to Philadelphia and could not leave a forwarding address.

No, I cannot explain it. My own eyes, however, saw both letter and envelope. Each bore a return address of Mary Jane Doe, 123 Main Street, Anywhere, USA 12345. There was one curious thing about the envelope. Although Miss Doe's ZIP was 12345, the postmark was that of the local downtown post office.

My young friend, too, has moved -- not, so far as I know, to Philadelphia - but I still ponder his parting gift. A query, of course. ''Gosh, you should have sent your money to Mary Jane. I bet she would have at least let you know it got there, don't you?''

Yes, son, I do. But life is more complex than you as yet perceive. Does it really matter that Sam is too busy to correspond? Someday when you are on my side of the desk you will understand that, as for Sam, no correspondence can be as welcome as a ''thank you.'' And if you are very lucky, you will not forget, as some of us do, that so long as somewhere in Anywhere USA Mary Jane (or John) Does can put aside their own frustrating concerns long enough to listen and respond to the whispers of preposterous curiosity . . . isn't that what really matters most?