The cylinder puzzle is a hands-on activity to model how scientists work. Students investigate an opaque cylinder threaded with ropes. By pulling on different knots and observing the results, they form and test hypotheses about what is hidden inside. The puzzle highlights observation, explanation, prediction, and the limits of scientific certainty.

See links below for instructions for construction.

1. Prepare a sealed opaque cylinder with four threaded cords crossing inside, each ending in knots.
2. Tell students the cylinder represents the “universe”—we can only observe from the outside.
3. Demonstrate by pulling knots in sequence (e.g., A-B-D-B, then A-D-B-D-A-B) and let students observe carefully.
4. Ask students to:
   • Record observations systematically.
   • Draw and explain possible internal arrangements (hypotheses).
   • Predict what will happen when a new knot (C) is pulled, then test it.
5. Encourage refinement of hypotheses after each test. Compare student models and discuss.
6. Optionally, allow opening the cylinder at the end to reveal the mechanism.

Mystery Tube - South East Physics Network:

📄 Cylinder Puzzle - Peter Dekkers: https://interactivetextbooks.tudelft.nl/showthephysics/demos/demo70/demo70.html

• Let students design their own hidden-string puzzle for classmates to test.
• Add more cords or make asymmetrical arrangements to increase complexity.
• Keep the cylinder permanently sealed to emphasize the limits of inference in science.

• None

• What observations did you record? How consistent were they?
• How many different models could explain the same data?
• Were your predictions accurate when tested?
• Can you be certain about what’s inside without opening the cylinder?
• How does this puzzle resemble real scientific work? (data collection, modeling, limits of certainty)