The trouble comes the first time you need a bag with you for something beyond the basic keys, wallet, phone combination. Something reasonable and indisputable - maybe you've started riding a bike and need a bag to keep your bike lights safe when you've locked it up. And then since you've got a bag anyway, you might as well bring that thin waterproof jacket, to be on the safe side. And the miniature puncture repair kit. That's sensible, and it weighs next to nothing. It's a slippery slope, oh yes. Soon you're used to carrying a bag with you. Suddenly, everywhere you go, you think before leaving the house, "what might I need?" The first few times the additions are still verging on reasonable. You add things you might very well use, even though you probably won't - sun cream, some mints, a swiss army knife. Every so often, someone will ask "does anyone happen to have a [thing that is in your bag]?" and you feel good that you did happen to have it and were Useful. Thanks to the positive reinforcement, things start to increase. Where once, you carried a single individually-wrapped sticking-plaster, you now have a small first aid kit. In addition to your book, you carry another book in case you finish that one. You give up on the limited functionality of the PDA and take the laptop around instead. Plus the mains cable, of course - it would be awful to run out of battery! Eventually you end up packing a towel and a Greek phrasebook and a tent and a camping stove just to go to Sainsburys.