Possessive forms
Incorrect . That room's windows are open.
Correct : The windows of that room are open.
Note: The possessive form is used for persons and animals only. But there are many exceptions to this rule.
For example:
It is a day's work only.
We took a night's rest.
We enjoyed a week's holiday.
Dhaka's population is very big.
That was the court's decision.
All the boat's crew were drowned.