11 RULES OF WRITING - RULE 5

RULE 5 -- To indicate possession, end a singular noun with an apostrophe followed by an "s". Otherwise, the noun's form seems plural.

RULE 5 -- EXAMPLES

Incorrect -- Correct

Though the lobsters claws were bound, the creature made a threatening gesture as they dropped it in the pot.
Though the lobster's claws were bound, the creature made a threatening gesture as they dropped it in the pot.

In a democracy, anyones vote counts as much as mine.
In a democracy, anyone's vote counts as much as mine.

There is a vast age difference between Victors mother and father.
There is a vast age difference between Victor's mother and father.

RULE 5 -- REFERENCES
More information about this rule is available from the following sources.

Online Resources:
The Elements of Style: rule 1

Printed References: (see References section)
Strunk and White: 1-1
Hacker: P5-a