Somewhere in an airport men's room, a grown man is aiming at a fly.

The fly is fake. It is etched into the porcelain. He knows that. He aims anyway.

This is one of the most quoted stories in behavioral economics, and it is a copywriting lesson in a trench coat.

Why does a tiny fly fix a big mess?

Here is the setup. In the early 1990s, Schiphol Airport in Amsterdam had a cleaning problem. Men missed. A lot. (Nobody puts this on the brochure.)

A sign is easy to ignore here. Nobody reads a paragraph while standing at a urinal. They stare at the wall like it owes them money.

So someone put a picture of a fly in the bowl. Now the eyes had a job. Aim at that.

The airport's manager, Aad Kieboom, told NPR that spillage dropped 80 percent. Other accounts say the cleaning bill fell about 8 percent. Both numbers are the airport's own tests, so treat them as a report, not a lab result. Even Richard Thaler guessed at how they measured it.

Who had the idea? Kieboom said it was a cleaner named Jos van Bedaf. He had served in the Dutch army in the 1960s and remembered a dot someone put in a barracks urinal. Same trick. Older fly.

What is a nudge, and why should a writer care?

Thaler and Cass Sunstein used the fly in their 2008 book, Nudge. A nudge is a small change in how a choice looks that moves people without forcing them. No rule. No fine. No lecture.

The fly did not say "Please be tidy." It gave a target.

Now think about your last email. Or your last sales page. How many "please" lines were in it? How many targets?

Most copy is a sign. Signs get ignored. Targets get used.

Where does it work in writing?

A button that says what happens next.

"Get my free checklist" is a target. "Submit" is a sign. One tells the reader where to aim.

A default that helps the reader.

Pre-pick the option most people want. Thaler and Sunstein wrote a lot about defaults. People often go with what is already chosen.

One next step, not six.

A page with six buttons is a bathroom wall. Pick the fly.

Where does it break?

A nudge can be used on people instead of for them. Hide the cancel link and you are not nudging. You are tricking. The fly works because the man is glad to be helped. Nobody feels cheated by it.

Test: would the reader be fine knowing what you did? If yes, it is a nudge. If you need them not to notice, it is a trap.

And one fly does not fix a broken page. If the offer is bad, a clear button just helps people leave faster.

Aim the reader

The fly teaches three things. Say it with me, at a urinal, quietly.

Give the reader something to aim at. A named next step beats a polite hope every time.

Put it where the eyes already are. The fly sits in the bowl, not on a poster by the sink. Your best line belongs where attention already lives.

Make the easy thing the right thing. If doing what you want takes one move, most people will make it. If it takes five, most will wander off and wash their hands.