English and ESL Instructor
Several students forgot their book today.
When it is hot, we need to drink plenty of water to keep our body hydrated.
In the first example, unless the students all use the same physical copy of a particular book, then it should be books. Similarly, in the second example, every person has a different body, so we should pluralize body:
Several students forgot their books today.
When it is hot, we need to drink plenty of water to keep our bodies hydrated.