@nhelen,
Quote:1) Great white sharks are dangerous to humans. Because they will attack without warning.
2) Great white sharks are dangerous to humans. They will attack without warning.
Both are grammatically correct, nhelen.
The first sounds a little odd out of context. But when there is context, it's not at all odd.
A: Great white sharks are dangerous to humans. [gets, for example, a roll of the eyes from someone else] Because they will attack without warning.