Set two cups a hand's width apart and lay a flat sheet of paper across them. Put a light book on top — it will collapse.
Ask how the paper could be made stronger without adding anything. Let them experiment: folding, rolling, layering.
Test each attempt with the same book so the comparison is fair. A folded concertina will eventually hold it, and finding that out is the entire activity.
The paper does not change, only its shape does, so the child discovers that structure creates strength. Testing each attempt against the same weight introduces a fair test, which is a real experimental concept arriving through play.