Avoiding Sentence Fragments

A sentence needs to have a subject and verb.
My cat has long whiskers.
If you have just a subject, or just a verb, you have a sentence fragment.
My cat.
Has long whiskers.
You can fix a sentence fragment by adding what is missing.
My cat has long whiskers.
Also, if a group of words does not express a complete thought, it is a fragment.
Because my cat has long whiskers.