Worm gears: why big turns small.
Turn a crank and count how far the gear moves. Then try to push the gear backwards.
Ages 8+Lessons
Gear trains: fast or strong.
Put a small gear next to a big one and see which way, and how fast, each one turns.
Sensors: how a robot sees.
Point a colour sensor and a distance sensor at things and watch the numbers change.
Route planning: fewest moves.
Plan a path on a grid, then find a shorter one.