If you want to test the hypothesis 'Boys are taller than girls', it is impossible to measure the height of every boy and girl in existence.
A sensible way around this would be to take a selection of boys and girls, measure their heights, and use this information to judge whether the statement is likely to be true or not.
This process is called sampling.
You should only use sampling when you have a large population.
For each of the following scenarios decide whether it would be sensible to use a sample.