My fax machine ran out of ink. I know this, because A) I received an illegible fax and B) the display on the fax machine said "replace ink cartridge".
So I went to Bluepoint to buy a new cartridge, and lo! the price is £18 which is not much less than I paid for the machine. No thanks. I have a cunning plan.
I took the old cartridge, made a little hole in it, and injected printer ink into it from a kit I bought last time this happened. It was easy to do, not much mess (I put down paper, and I'm sure the black will come off my thumb in a week or two). I put the cartridge in, and the display continued to say "replace ink cartridge". So I swore at it a bit, and that seemed to do the trick. I copied a page to make sure it's working, and I'm all set now. These ink kits cost about £10 and there's enough ink to repeat this a few dozen times.