Both ends of cm 2 are lit together, which will take half an hour to burn.

3. Take out 6 liters of water from the 6-liter kettle and pour it into the 5-liter kettle, leaving 2 liters of water in the 6-liter kettle. Pour 2 liters of water into a 6-liter kettle to get 6 liters of water, pour it into a 5-liter kettle, and leave 1 liter of water in the 6-liter kettle, pour 1 liter of water into a 5-liter kettle, and leave 3 liters of water in the 6-liter kettle.

4 First time: 5 in each pair, and 5 light ones are defective.

Second time: those with defective products are divided into three groups: 2, 2, 1. If 2 and 2 have the same weight, 1 is defective.

If their weights are different, separate the lighter groups.

To sum up, weighing at least three times can ensure that defective products are found.

Science: In the 2nd century A.D., Ptolemy, Greece, established the geocentric theory of the universe, and he mistakenly believed that (the earth is the center of the universe).

2. The Polish astronomer (Copernicus) wrote his masterpiece "The Theory of Celestial Motion" and founded the "Heliocentrism".

Foucault proved (the rotation of the earth) by experiment.