A favorite espionage tool, at least in the movies, is invisible ink. A spy writes a message that can't be seen by the naked eye, but which magically appears when another operative applies a chemical spray or fumes.

Now the big Xerox Corporation, which makes some of the world's best-known copying machines, has come up with an invisible-ink concept in reverse. Using a special kind of paper and a modified printer, the Xerox system lets you create normal-looking documents from your computer, documents that will go away, disappear, poof! after a day or so. You can even erase them immediately by running the paper through the special printer a second time.

The idea is that this erasable paper can be re-used, over and over again, saving money and cutting down on refuse in wastebaskets and recycling bins.

This inspires all kinds of imaginative ideas. You could print checks on this self-erasing paper, for instance, and then run out and buy lots of products. Then you could admire this brilliant idea from your jail cell.

Politicians and diplomats are always looking for what's called "deniability" for their actions. What better way to get it than to send messages that disappear in a few hours?

Want to fire off a nasty note to a company or a neighbor or a boss? "What nasty note?" you can ask the next day.

Of course you wouldn't want to write your "ten things I need to do" checklist on this paper. Or your mailing list. Or an important phone number.

The cost-saving angle of this reusable paper won't work in offices where people are too busy or lazy to gather up and neatly re-stack all the newly erased sheets. But the frugal person who's already feeding the blank backsides of used paper into the copy machine will be thrilled by Xerox's newest breakthrough