Factual writing generally calls for “complete” sentences, which means sentences that contain at least one independent clause. Anything less than an independent clause—a dependent clause or prepositional phrase or non-finite verb phrase—that’s punctuated as though it were a sentence is called a fragment. Fragments come just before or after the clause they should be integrated with. They are easy to spot if you read aloud for sentence rhythm:
ERROR: Ducks usually migrate in flocks of friends. Because relatives tend to screw up the formation. [The dependent “because” clause modifies migrate, the finite verb in the preceding clause. No punctuation is suitable between the two clauses:]
Ducks usually migrate in flocks of friends because relatives tend to screw up the formation.
ERROR: Elephants are terribly ticklish. As I proved by blowing in Jumbo’s ear. [The dependent “as” clause modifies the finite verb are, and so should be part of the same sentence.]
Elephants are terribly ticklish, as I proved by blowing in Jumbo’s ear. [For the comma between clauses, see Introductory Phrases and Clauses.]
ERROR: Ms. Beebs coiled Fang counterclockwise. Absent-mindedly looking out of the window. [The non-finite verb phrase beginning with absent-mindedly modifies Ms. Beebs. The proper position for the phrase is at the beginning of the sentence.]
Absent-mindedly looking out of the window, Ms. Beebs coiled Fang counterclockwise.
ERROR: They want to invite Madame Coil. The cobra voted best hisser of 1977. [The noun phrase beginning with The cobra voted best hisser is in apposition with Miss Hiss and needs to be in the same clause.]
They want to invite Madame Coil, the cobra voted best hisser of 1977.
ERROR: He coiled Fang counterclockwise. And put him in his basket upside down. [Coiled and put are coordinate finite verbs, both part of the same independent clause.]
He coiled Fang counterclockwise and put him in his basket upside down.