I THOUGHT pregnancy was the thing that rotted our brains. We're always hearing about how, the instant a woman conceives, she turns into an air-head.

Well, now it's holidays. You wouldn't think it -- I always assumed they recharged your batteries -- but according to a leading psychologist, time off dulls our ability to think.

After three weeks off (not many people get that much time off in one stretch), our brains shrink and our IQ falls by 20 points.

I don't agree. Many a time I've been close to brain dead at work, barely capable of writing my own name. I've gone on holiday and come back ready to rattle out a 30-page supplement on the impact of the Euro on Third World financial markets.

No, in my opinion holidays don't rot our brains. But I'll tell you what does:

Watching an episode of Big Brother.

Incredibly, this second lot of inmates are even more tedious and uninteresting than the first. I'm all for watching people lolling about on a sofa -- the Royle Family do little else -- but the characters are likeable and their banter witty. This lot are painful to watch. There's more to stimulate the grey matter in an episode of Postman Pat.

Trailing around packed shopping malls (those horrible indoor ones) on a weekend.

They're airless, squeaky clean and claustrophobic. I visited one recently with a friend -- who was infinitely more tolerant than I am -- and emerged gulping like a koi carp. My head was throbbing and I had trouble remembering the colour of my car, never mind where I'd left it.

Visiting mobile phone shops and computer software stores.

There's nothing worse for dulling the brain than listening to a spotty, greasy-haired youth (average age 17) wearing a beige suit three sizes too big, drawl on for hours about the merits of the latest Nokia handset. In this situation -- as I recently found myself -- the vision begins to blur and it becomes difficult to string together a sentence that doesn't sound like a verbal text message.

Visiting car showrooms.

Similar to the above. They all look and feel the same. Even the sales people are cloned. After two or three they all begin to melt into one, and there's nothing worse than feigning interest in a subject you know very little about.

How come sales staff never accept the fact that you just want "something small and reliable to travel from A to B?"

You always have to sit through the spiel -- the number of revs per minute, the turning circle, the integral cigarette lighter and barbecue.

Brain malfunctions are guaranteed. I dread going so much that I'm hanging on to my rust heap until I have to drive it Flintstones-style.

Looking after young children.

Not that it isn't enjoyable. It is, 10 per cent of the time -- it's the other 90 per cent that knocks your mental state for six.

My friend and I -- both mothers -- spotted an X-Men number plate on a car the other day, then proceeded to (quite seriously) discuss the talents of the various X-Men characters.

We scared ourselves. As two adults we should have been talking about the Tory leadership race or at the very least, new products on the Clinique counter in Boots. But being with children makes you forget the "adult." That part of the brain lies dormant.

Reading women's magazines.

"What makes him fancy you?" "Why you fancy him?" "3,000 ways to get rid of cellulite," "Firm breasts in 30 minutes." The formula never changes. Are the editors actively trying to kill off women's brain cells?

I'm sure there are many more brain-rotting pastimes, but I can't think of any. My brain has had enough.

That's reminded me of another brain damaging activity, perhaps the worst culprit of all -- work.