To say “A is stronger than B”, the comparison must be fair. That means changing one thing at a time and keeping everything else the same.

Suppose you test strips of paper, cling film and thin cloth to see which holds the most weight before it tears. A fair test needs:

• The same size of strip (same width, thickness and length between the clamps), because a wider strip carries more.
• The same way of loading: add weight slowly in equal steps, not suddenly.
• A clear definition of failure: for example “the strip separates into two pieces”, not “it looks weak”.
• The same conditions: temperature and humidity affect many materials.