If a stranger thrust a paper cuplet of gloop in your face and said, "Good morning sir! Swallow this," you would recoil. One moment you are drifting along, contemplating your own perfect thoughts, the next you have the pungency of false cheese in your nostrils and a sense of your personal space being not just invaded but colonised, as a total stranger is leering and suggesting you ingest the substance in their little paper pot.

You would leap back. You might dash the vile container from their hand. You might reflexively punch them in the jaw, without fracturing anything and giving no grounds for any kind of lawsuit no matter what they bleated as they lay on the concrete surrounded by scattered samples.

You would not, under any circumstances, say, "Oh, fine, what is it," and slurp down a big mouthful.

Would you?

Yet place this same stranger under a striped canopy bearing the logo of a soon-to-be-national brand of soft cheese or yogurt drink or frozen custard and suddenly passers by are huddling round, eager to gulp down whatever is in the proffered cup.

It's free, you see. How can anything free be bad?

(International invitations to money making schemes, the flashing sex aid ads in your email inbox, your first hit of heroin...)

And the stranger's perky uniform. So clean and white. This person appears practically a clinician with their faintly nurse-like tunic and cheery name badge. And a nurse would never make you swallow anything nasty, hmmn?

If I wanted to mass-poison the denizens of a small town I would show up in a truck smothered in colourful decals, and set up a stall under a giant banner that pronounced Free Diet Chocolate Yogurt Ice Cream, and watch as my victims doomed themselves. I might write down a few of the forced compliments at my gruesome concoction while I was at it. "Mmn, real different." "Wow, that is certainly something."

Yes, it is something all right, sir: half-rotted milk laced with cyanide, and you have just snuck a couple of extras when you thought no one was looking.

This is the nature of trust and greed. A stranger means danger, but a stranger with a badge and a tray of gratis goodies, that is nearly the same as a friend. A friend on commission.

Watson says I need to calm down and also to write a letter of apology to the supermarket, the woman giving away the samples, and possibly the USDA.

She has placed a cup of tea beside my desk, without being asked.

"What is it? Are those twigs?"

"Just drink it."

"There is no point in apologising to those people. Anyway, it was probably the most exciting thing to happen since they put flower holders on the trolleys."

"The cart you rammed the stall with could have used airbags as well."

"Nonsense, the child was fine. He or she loved it. Why is this tea purple? Are those pieces of damson?" Why does Watson insist on these mystery beverage-based gifts?

"Just drink it."

"I am not some control group in your ongoing experimentation with herbal concoctions." Although it does save me having to get up and go to the kitchen. And Watson's creations are generally fairly palatable. "What are these bits floating on the surface?."

"Just drink it."

I drink it. After all, you have to have trust.