Realizing that most students associate computers with big, expensive, shiny boxes that they're not allowed to tamper with, I thought I would try to convince them otherwise.
1. I kept the RasPi in its little white box and told the students to ask me yes/no questions about what it was. They eventually got to calculator at which point I opened the box and told them that this was a computer. They appeared fascinated by its appearance as we passed it round and I told them it was a computer capable of doing all the things we do on the computers at school. Including running Scratch!
2. Someone then pointed out that at the moment we couldn't type anything into the computer, like we'd been doing in English. I agreed and so asked the class what we needed to be able to use it.
I then sent them off into groups to discuss this and then write down or draw a list of stuff we would need.
3. After five minutes we got back together and I asked them what we would need. As they mentioned things I pulled them out of my bag or detached them from the classroom PC. In this manner we ended up with: keyboard, mouse, monitor, power supply and plug. The one thing they didn't get, perhaps understandably, was the SD card on which to store our work (and operating system!)
4. We then set it all up and got Scratch working! We played around with it a little and saw that it was pretty much like the version we had been using on our school PCs.
5. I told them the price and explained that it is a cheap way for them to get into programming.
6. Next I intend to give them details of where and how to get one.