Until the early part of the last century, farmers used horses or oxen to pull carts and help plough the fields. Now tractors are used to do this and for lots of other purposes as well. Could you make one?
Levels of difficulty:
Level 1 - Make a simple model of a tractor. Level 2 - As level 1, powered by a battery motor. Level 3 - As level 2, with a plough on the back, and the plough the sand in a sandpit.
Equipment: KNEX set including small and medium wheels, plus a battery motor and (for level 3) a sandpit.
Approach: What does a tractor look like? What type of wheels does it need? Where does the driver sit? For level 2, how can you make the motor drive the wheels? How will the motor be fastened to the body? For level 3, what is the purpose of a plough? How can you make one out of KNEX? How will it attach to the back of the tractor? Does it work when tested in a sandpit?