After teaching students some basics (stage, sprites, costumes, loops, movement, ask/answer, broadcast/receive, variables), I challenged them to create their own projects.
"May I read your mind?" We play a binary guessing game and read their minds. After explaining the game, we program it in Scratch, starting from a project that only requires a few key additions.
This is an over simplified representation of how poisons came to become common ingredients in American food. There is an interactive portion at the end.
I work at a special education school in Queensland where I am using Scratch more and more. This website aims to direct other teachers to this wonderful world, with a few projects to get them started.