You can use Sketchpad to model real-world objects such as wheels and linkages.

• "Constructing a Pantograph", from Geometry Activities for the Middle School, ©1999 by Key Curriculum Press
• Turning Wheels, from Key Curriculum Press
• Constructing a Ferris Wheel, by Jim King
• Objects to try to construct:
  • An ironing board
  • A clock, complete with big, small, and second hands that rotate at the correct speeds, relative to each other
  • A bicycle, complete with pedals
  • A basketball going through a basket