I mentioned yesterday that I had seen lots of bus shelters in Edinburgh this year adorned with a notice that read: "Beware - the top of this bus shelter has been treated with anti-climb paint!".

What I didn't say, because I didn't know, is what anti-climb paint is. While it is always nice to be in at the birth of a new word (can't find "anti-climb" in any of the dictionaries I have looked at), it's also irritating not to be able to visualise this new stuff, or how it works. You can't easily examine the tops of bus shelters except by climbing up there, and that is what anti-climb paint was invented to prevent in the first place.

"All right! Come on down! It's against the law to climb on bus shelters!"

"I wasn't climbing the bus shelter, officer. I was just trying to see what anti-climb paint looked like."

"That's a good one."

"It's the truth. I'm an investigative journalist, and I owe it to my readers to find out ..."

A friend of mine who has seen the stuff says that it is like paint except that it is very thick and clingy, and doesn't dry properly, but he isn't sure if it is designed to make a mess of your clothes or to stick to you. My feeling is that it wasn't invented at all. Things like this don't get invented. They are usually accidents.

For instance, I remember reading 20 years ago about some chemical company that had been trying to create one thing, a cure for athlete's foot or something similar, and had come up with something quite different - a cream that turned things brown. That's all it did. Turned things brown. It was useless. They were about to throw it away when someone had a brilliant idea. It was not quite useless. They could market it as a suntan aid. They did, and made a fortune.

Similarly, you can imagine a paint firm coming up with one that never dries, and has an unpleasantly adhesive quality, and the paint is about to be ditched for ever, when some keen young executive cries: "Wait! I have a chum working for the Lothian Region who tells me they are desperate for a way of stopping people climbing on to bus shelters! This could be the answer!"

Months later, everyone is either very happy, very rich or stuck to the top of a bus shelter in Edinburgh.

Perhaps in the early hours of the morning the Edinburgh police send round a bus shelter recovery team. Mission: inspect the top of all bus shelters. Aim: locate and remove all persons sticking to them, and take them to hospital to be treated for exposure.

"Stuck out all night, were you?"

"Yes, doctor."

"What mountain were you up on?"

"It was not exactly a mountain..."

"Was it very high?"

"About 10 feet."

"!!!!!"

"Well, you see, I'm an investigative journalist, and I wanted to know what this anti-climb paint on bus shelters was like..."

"So you've been stuck up on a bus shelter all night?"

"Yes."

"Did you not call for help?"

"I banged on the roof of the bus shelter, but the people below me in the bus queue thought it was the people upstairs having a bit of a wild time and banged back."

Potentially humiliating. But not as bad as another humiliation which could take place a quarter of a mile away. In Rose Street there is a large climbing and walking shop called Tiso, and on the third floor, where they sell all the climbing and walking boots, there are various places where you can test your footwear. There is a ramp on the floor down which you can walk to see if the toes behave properly when you are going down a steep mountain. And there is a wall nearby which has plastic imitation rocks screwed to it, so that you can climb up it with your new climbing boots to test their holding qualities. (The imitation rocks were probably invented in the same way as the suntan aid or the anti-climb paint - by ignorance.)

Beside the wall, there is a notice that I have never seen before in a shoe shop, saying simply: "Customers use the climbing wall at their own risk." You can see the point. No shoeseller wants a serious climbing accident in his shop. But then no customer wants to be taken to hospital this way ...

"Hello, it's you again. Been stuck on another bus shelter?"

"No, doctor. It's a climbing accident this time."

"A climbing accident? On a bus shelter?"

"No, doctor. In a shoe shop, actually ..."