Well I don't have a clue either, but now I know what a demijohn is, and I want one.
I'm due a birthday present, so I dropped a hint yesterday about a homebrew kit, and the answer was not a chance. That's not the answer I was hoping for, but I'll order a kit online and play dumb. I'll almost certainly need a demijohn, too

.
I have a cunning plan, though. My uncle was a brewer back in the day for a microbrewery. Dempsey's was their name, I think. I remember he used to pick my aunt up from our house when they were courting, she was living with us at the time, in this old style van with Dempsey's plastered down the side. A bit like this one.
I've no doubt there was plenty of courting going on in the back of that thing. Anyway, I digress. He's just retired and he has ants in his pants, so I started dropping little hints over the last year or so that he should get back into brewing again and set up his own microbrewery. He has plenty of money. And I think these seeds have found some fertile ground. He's been researching the cost of equipment, he told me, and has gone on a couple of brewery tours to see what has changed in the process, etc. Next time I see him I'll drop the idea that I could help him if he needed it. Inception.