Fair warning: If you wake up slowly, have a short fuse in the morning or find yourself unable to function well until after your second cup of coffee, skip this first item.

If you persist in spite of my warning, and you become irritated as a result, blame Molly T. Guertler of Arlington, not me. Molly wrote:

"Try this on your readers. Tell them to write down their house number, double it, add five, then multiply by 50. When they have done this, tell them to add their age, the number of days in the year (leap years don't count), and subtract 615. The last two numbers of their answer will be their age. The others will be their house number."

I tried it. It worked.

If you try it, you will also find it works - unless you're 100 years old or more.

Once you discover that it always works for anybody 99 or younger, perhaps you'll be curious to know why. I think I can explain, even to people who don't know algebra.

Any address will work, so let's say your house number is 1234. When you "double it, add five, and then multiply by 50" you are, in effect, multiplying your house number by 100 (first you multiplied by 2, then by 50) and what's more, you are adding 250 (50 times the 5 you added to your number.)

Let's pause to check what we have done so far. Double 1,234 is 2,468. Plus 5 makes it 2,473. And 50 times 2,473 is 123,650. Is 123,650 the same as 100 times 1,234 (123,400) plus 250? It is. We can proceed.

Add your age. Mine is 39, and if you're willing to believe that story I'm willing to believe that your age is also 39.

Now add the number of days in the year, 365. Subtract 615, and you're finished.

What have you really accomplished with all this hocus-pocus?

When you added 365 and subtracted 615, the net effect was to subtract 250, which is precisely the amount you added earlier. Remember? You multiplied your house number by 100 and at the same time added 50-times-5, or 250. Therefore when you subtract 615 at the end of this exercise, you wash out the "plus 5 times 50" and the "plus 365."

So all you have really done while going through all this busy work is multiply your house number by 100 and add your age. If you are under 100, the result will be your house number followed by your age.

In this case, you will recall, we had checked our number work to a subtotal of 123,650. Now finish the arithmetric for this example. To 113,650, add your age, 39, and you have 123,689. Now add 365 and you're at 124,054. Subtract 615 and, Presto! The answer is 123,439. Don't let the comma in 123,439 deceive your eye. When the number is written as 1234-39, the last two numbers are your age, the others are your street address. Ain't science marvelous?

If you are now tempted to show off your new-found knowledge you can ask a stranger to write all these numbers and calculations on a piece of paper, out of your sight. Then, after he tells you only the final figure at which he has arrived, you can astound him by telling him his house number and his age.

The only trouble with trying to show off with the address-age trick is that most people can't do this many calculations without making one or more mistakes.As a result, you may be led to believe that the house number for a 1,234-year-old man is 39.

The safest way to play clairvoyant may be to lend the stranger a pocket calculator or an abacus. And even that isn't foolproof. I have an ominous feeling that today, of all days, my cordless abacus has betrayed me. I hope not.