No, I haven't misspelled the Egyptian capital! I have recently been introduced to kairo by some elementary school children. Basically, it looks like a tea bag, and is filled with iron shavings. So they come individually wrapped (so Japan!) and when you take it from the plastic and put it in your pocket for a couple of minutes it heats up. It stays hot for a few hours... it's like carrying a mini hot water bottle in your pocket!
So most days at school I have one that I carry around! Apparently, the Junior High kids are not allowed them, but a few students have gestured that they are carrying kairo too! Apparently you can get kairo that you stick to the inside of your clothes (if you stick it to your skin you may get burnt) I haven't tried them yet, but I have no doubt I will progress to those very soon!