Cardboard tubes, tape, and gravity. Children build a track that carries a marble from the top of a shelf all the way down to a cup on the floor. The marble really does make it — if the track works.
The iteration loop is the fastest yet: place the marble, watch where it fails, fix that spot, extend, repeat. A pair might go through 15–20 test cycles. The Share-out is theatrical — the whole group watches each marble run, counts how many sections it passes through, and erupts when one makes it all the way.