My brain tells me £99.99 is 1p less than £100. But my heart only sees the first nine, and would round down to £90, writes LUCY KELLAWAY

LAST WEEK's big news at my office was that the tills in the canteen were on the blink and could no longer read our vending cards. So in order to buy, say, a large latte and a lemon muffin, we had to open our purses and find £1.80 in coins instead.

This caused quite a stir. I objected not only because any change to routine is abhorrent but also because the business of having to hand over real cash suddenly made my morning treat seem rather more expensive than usual.

Such irrationality is shameful. Under the vending card system, I would put £20 on my card, which then would be swiped every time I bought anything. In effect, I was not only giving the canteen interest-free credit, but was also consuming more Maltesers than I otherwise might have, under the misapprehension that, once I had charged my card, my purchases were somehow free.

Last week's pay-as-you-go system was clearly better for me: I can understand that. I just can't quite accept it.

My shame at this financial nuttiness has been assuaged a little on hearing the following story from a colleague. One evening last week, he was waiting for a train home when he dropped 5p on to the platform. He bent to retrieve it, but found that picking up a small coin from the ground was not easy when you have large fingers and short nails. He groped and fumbled and, as he did so, the train came in. For a few agonised seconds he was torn between train and coin; he wavered and, just as the doors were closing, he flung himself into the carriage. His decision was painful in body but worse in mind: indeed, the fact that he'd left his 5p behind on the platform was still hurting him enough 24 hours later for him to feel the need to tell me about it.

There is something comic (or sad or worrying, depending on one's point of view) about two financial journalists with 35 years' experience between them sharing such stories. Our newspaper daily chastises people who get themselves into trouble over money: the homeowners who have borrowed more than they ought, and the reckless lenders who have lent more than was wise.

Yet neither borrowers nor lenders were illogical. They were motivated by need and greed and the general idea that what has happened in the recent past will go on happening.

By contrast, my irrationality over the most basic rules of money is harder to fathom.

My brain tells me that £99.99 is 1p less than £100. But my heart only sees the first nine, and would round down to £90. In the tussle between heart and head in controlling my wallet, my heart, as it were, wears the trousers. Never mind the maths A-level I once got, nor the degree in economics - none of it registers.

The second nutty principle is that the more arm's-length the transaction, the less the expenditure counts. Cash is more painful than credit card, which is more painful than direct debit. Indeed, the latter stops being painful at all if you barely glance at your bank statement. Then it is almost free.

A third area of monetary nuttiness is the in-for-a-penny-in-for-a-pound principle. Once I have got over the pain of a first large expenditure on something, subsequent ones hurt much less as long as no time has expired in between. If I've already bought an expensive jacket, I might as well follow it up with some expensive shoes and trousers in the same afternoon.

Then there are the specific areas of irrational stinginess. My parents used to drive for miles to get to a garage with petrol that was 1p cheaper. A friend will not buy a book in hardback. My miserly obsession is taxis: I'd rather walk for hours with a heavy suitcase than get in a cab.

Free offers and bargains warp my reason too. On Amazon the other day, I bought some books and needed to spend an extra £4.20 to get free delivery. There was nothing else I really wanted, so I chose a maths workbook that my son actively did not want, to avoid paying £3 postage. Then I decided that actually I would like the books to come the next day, so ditched the free delivery option and paid to have them all sent express - maths workbook included.

Supermarkets are temples to consumer irrationality and I am a devout worshipper there too. I don't buy Pringles as I find them addictive and fattening. But last week they were three-for-two so I changed my mind and opted to buy two fattening tubes in order to get a third fattening one free.

It isn't just spending money that makes me lose my reason - earning it confuses me too. A year ago I agreed to give a lucrative speech and, once the date was booked, I felt rich and spent the fee. On delivering the speech, I felt I had actually earned it, so spent it again.

Subsequently the company got into financial difficulty and failed to pay me, which was a shame as I had already spent the money twice. Six months later, out of the blue, the money arrived. As I had mentally written it off, this counted as a windfall, so I spent it all over again.

Companies should take note from these nutty examples. Often they try to get employees to treat company money as if it were their own.

This is a bad idea. In the struggle between the educated economist in my brain and the financial cavewoman in my heart, the economist is always in control when I am handling other people's money. The mad cavewoman only starts to stir when the money in question is my own.