Here is a tempting plan: to drive a square one metre on a side, command 'forward for 5 seconds, turn left for 1.5 seconds', four times. We worked out the formulas, so the robot should draw a perfect square. Run it on a real robot and the shape almost never closes. The robot ends up a few centimetres from where it started, or worse.

This kind of control is called open loop: the commands are worked out in advance and sent out, and nothing measures what actually happened. The opposite is closed loop, which uses feedback (the loop from the first lesson). Open loop is fine when the world is predictable. Robots rarely live in a predictable world.