in the States
it is called 'Trafficking and Trading" by the officers (and is illegal) but it is called "fishing" by the inmates. They pull threads out of the sheets to make a rope. Sometimes they might have a stolen paperclip or staple on the end as a hook, sometimes it is just a piece of tape or a knot for weight. The inmates will somehow toss the end of the string (I never saw it from the inmate's view) under their door to another cell. If they are fishing for something, the inmate that is trading with them will toss out a line to catch the 1st line, pull it in, and attach whatever they are trading. They become very adept at this and it is quite a sight to watch all the lines going back and forth across a pod or between the rows.

During lock-downs, the inmates are all fed "johnnies'...which is a sack lunch with a sandwich or two, cereal, and milk. Once the officers have passed out a johnny to each inmate, if there are any left, they will usually put the left-overs either on a table (which only the best at fishing can reach) or on the floor. They then yell out, "if you can fish it, you can have it" and leave the pod. When the officer returns a bit later, they will all be gone.
( flirting with badgers What? It sounded good at the time., Tue 1 Apr 2008, 1:22, Reply)