The first thing out of the envelope was a $20 bill. For some reason, I looked at it in puzzlement for a while.

Subconsciously, I was aware that there was something wrong with the bill, but that awareness failed to surface or take concrete form.

When I unfolded the letter that had accompained the twenty, I found it was from H. Rodgers Gore of Cheverly. He wrote:

"Somebody passed this phony $20 bill to me. I didn't notice it until I tried to use it. Banks will replace mutilated bills if the two serial numbers are still intact. I sure do hate to get 'taken' in this way."

I picked up the bill and looked at it more closely. It had obviously been torn and pasted together again, but everything else about it seemed in order. "The United States of America" was authentic, "Federal Reserve Note" couldn't have been more genuine, the Treasury seal, the signatures, the picture of George Washington, the . . .

The picture of George Washington for heaven's sake? Good grief! A twenty is supposed to show Andy Jackson. Washington is on the $1 bill.

Somebody had altered a $1 bill by putting a "20" in the place of the digit "1" that is supposed to be in each corner. The scam works because most Americans do not look closely at currency.

The altered note plainly said "One Dollar" in large letters, right under Washington's picture, but when I took it around The Washington Post newsroom one night and asked for change, nobody spotted it for a phony. We've all become accustomed to looking at the corners only, and even a bill that appears to have been accidentally torn and subsequently pasted together arouses no suspicion.

The law forbids altering U.S. currency, of course. One who "raises" a Federal Reserve note is guilty of a felony -- even if he has not yet tried to pass the bill.

The Secret Service, which has the responsibility for protecting the integrity of our currency, has a surprising amount of success in tracking down and jailing people who think they have found an easy way to make money without working.

Nevertheless, Secret Service agents told me the public could help greatly by heeding these reminders:

Be aware that there has recently been an upsurge in the circulation of altered currency.

Be advised that such phony bills are most often passed to busy clerks and cashiers -- especially those in fast food restaurants and other retail establishments during their peak periods.

If you accept an altered or counterfeit bill, you're the loser. Neither your bank nor your government will reimburse you for the bad bill that's taken from you.

Be on guard against accepting a phony bill tendered by anybody. Even a person known to you to be honest may have been hoodwinked by a swindler. Your best friend may unwittingly be carrying a bad bill.

If you are a young and inexperienced cashier, keep in mind that you're a special target for the crooks. They like to give their business to green kids.

If anybody tries to give you a mended bill in change, look it over with special care. That "accidentally torn look" should set off loud warning bells in your head.

Know your money, and don't depend on the corner digits alone. On every bill, the correct amount is also spelled out in words that appear right under the picture. It is a good idea to memorize whose face goes with which denomination. Washington is on the one, Jefferson on the two, Lincoln on the five, Hamilton on the ten, Jackson on the twenty, Grant on the fifty and Franklin on the hundred. Without Andy Jackson's face on it, my phony twenty is worth $1.

One Secret Service agent told me, "For some reason, Washington has far more raised-note operators than the national average. This area just seems to be a hotbed for these people.

"They think they have found a fool-proof system, but we're on to all their tricks and systems. We catch them constantly, in fact we've caught many of them several times -- even while they're on parole for a previous offense of the same kind. I arrested one subject recently who had been altering currency ever since he was a juvenile, and getting caught ever since he was a juvenile.

"we'll never have enough man-power to put them all in jail. What we need is heightened public awareness that these people are putting our entire monetary system into jeopardy. They can be stopped only be massive public cooperation. Please ask your readers to familiarize themselves with our currency and to take a few seconds to really look at every note that passes through their hands."

All right, class. Here's a quickie quiz: What's the clue that should immediately put you on guard? Right! It's that "accidentally torn and patched up look." Don't fall for it.