Everywhere in Buenos Aires, there's someone trying to sell you something. On the subway, invariably there's a team of vendors selling 'Guia T' or pens that write in ink that can only be seen with ultra-violet light, or colourful little cards with platitudes on one side and a calendar printed on the other. Or hairclips. Or kites. There's a guy who's always selling tissues in the subway tunnel between Linea A and Linea C, "dos paquetes, un peso". Repeating the same phrase over and over, all day, day in day out, he has a particular and pleasing pronuciation of the word "pesso", emphasising the 's' with a rush of affrication.

In some cafes, vendors will come in and leave a sample on each table, and come around and collect them again or take money from those who take a fancy to the torch or pen or hair-clip. I was initially surprised at the number of people who actually buy these items. But it took me very little time to discover an occult secret of BsAs - if someone leaves something on your table, you will need it within 24 hours. The first thing someone left on our table was a very comprehensive looking interchangable-head screwdriver. I scoffed inwardly at the prospect of trying to sell something like that in a cafe. Then the blind in our apartment broke, and I had to buy a screwdriver from the the supermarket to fix it. After this many pens were left on my table mere hours before I wanted one, and packets of tissues galor presaged allergy attacks.

I confessed this superstition to a friend visiting from Australia, when there was a flashlight placed on the table of the cafe we were having lunch in. She gave me an understandably condescending laugh. That night, there was a power cut in her hotel and she had to put on her makeup by the glow of a cigarette lighter.

These are generally rather ordinary objects, but they seem to give, if you squint, a keyhole glimpse into the immediate future. I'm eager to find out what happens to me the day after the day that someone puts one of the ultraviolet pens on my table. What could it possibly be?