There are so many good reasons to follow Roger Ebert on Twitter. Here's just one post.
I could never reason out how a sewing machine worked. On Twitpic
I'm thinking how I'd introduce a 4/5 grade physics motion, design, and mechanics unit with this illustration.
First I'd ask kids to imagine how a sewing machine works. I might give them a needle and thread and scraps of cloth. I'd ask them to draw a diagram or act out what they think happens in the machine. I'd ask them to share their theories and speculate some more. Then I'd show them thread components, (the bobbin and the thread coming off the spindle and through the needle) and have them fuss some more. Then I'd show the machine in action and open the little door that holds the bobbin while the machine is running. Surmise, surmise, surmise. Ultimately I'd show them RE's fabulous illustration.
There are so many key concepts are in play here! Things you could return to, too, like tensile strength. It would make a better entree into physics than the Rube Goldberg approach I've seen too many times. That's funny but too explicit-- lots of what goes on in machines is covert, which is a great reason to use them to get kids theorizing.