Possessive nouns usually indicate ownership:
Frequently, however, ownership is only loosely implied:
If you are not sure whether a noun is possessive, try turning it into an of phrase: the roots of the tree, the work of a day.
Exercises:
The apostrophe 1
The apostrophe 2
Related topics:
Joint possession
Possessive compound nouns
Adding -s to an indefinite pronoun
Adding -s to a noun for possessive
Adding only an apostrophe to a noun for possessive