Here's why:
* It's a common noun: Biology refers to the general study of life, not a specific name or title.
* It's not a proper noun: It doesn't refer to a particular person, place, or organization.
However, there are a few exceptions:
* Titles: If "biology" is part of a title (like "Biology 101" or "Department of Biology"), it should be capitalized.
* Specific instances: When referring to a specific branch of biology, like "Molecular Biology" or "Marine Biology," the second word (the branch) should be capitalized.
In short, you should only capitalize "biology" when it's part of a title or a specific branch of the field.