This experiment will let you compare the strength of carrier bags provided by different retailers.
Plastic bags from different supermarkets
Four G clamps
Four wooden battens
Cut a 30cm square from the plastic bag.
Place one edge between two wooden battens and clamp to the edge of one table firmly with two of the clamps.
Repeat with the opposite edge and clamp to the other table.
Measure how far the tables are apart and make sure this is the same for each test.
Add masses 100g at a time to the centre of the plastic sheet until it breaks and record the mass used.
Repeat for each plastic bag from the different supermarkets.
Be careful to stand well back when adding masses as the plastic may break unexpectedly and the masses could fall on your feet. Also make sure you are using heavy enough tables that will not be pulled over by the mass in between them.