Rhetorical Question

A rhetorical question is a figure of speech in the form of a question that is asked in order to make a point, rather than to elicit an answer. Though classically stated as a proper question, such a rhetorical device may be posed declaratively by implying a question, and therefore may not always require a question mark when written. Though a rhetorical question does not require a direct answer, in many cases may be intended to start a discussion or at least draw an acknowledgement that the listener understands the intended message.

A common example is the question "Can't you do anything right?" This question, when posed, is intended not to ask about the listener's abilities, but rather to insinuate a lack of the listener's abilities.

Although sometimes amusing and even humorous, rhetorical questions are rarely meant for pure, comedic effect.
Different forms of Rhetorical Questions

Negative assertions
Often a rhetorical question is intended as a challenge, with the implication that the question is difficult or impossible to answer. Therefore the question functions as a negative assertion. Such negative assertions may function as positives in sarcastic contexts. For example, when a speaker repeats a statement reported to have been found true and adds a sarcastic Who knew?, the question functions as an assertion that the truth of the preceding statement was - or should have been - already utterly obvious: Smoking can lead to lung cancer. Who knew?!"

Rhetorical questions as metaphors
One common form is where a rhetorical question is used as a metaphor for a question already asked. In the vernacular, this form of rhetorical question is most often seen as rhetorical affirmation, where the certainty or obviousness of the answer to a question is expressed by asking another, often humorous, question for which the answer is equally obvious;.
Other forms

Sometimes the implied answer to a rhetorical question is "Yes, but I wish it were not so" or vice versa:

Another common form is the expression of doubt by questioning a statement just made; for example, by appending the following to a sentence: or did he?, or is it?, etc.
The butler did it... or did he?

It is also common to use a rhetorical question to bring an end to a debate or to finalize a decision. For example, when internally deciding whether to perform an action, one may shove aside the dialogue with a simple, "Eh, why not?" or "What the heck?"
Proper Grammar used with Rhetorical Questions

Depending on the context, a rhetorical question may be punctuated by a question mark (?), full stop (.), or exclamation mark (!), but some sources argue that it is required to use a question mark for any question, rhetorical or not.

Rhetorical questions may be signaled by marker phrases; questions that include "after all", or "by any chance" may be intended as rhetorical.

Written lists of rhetorical questions within a sentence require question marks, but do not require quotation marks. "Would he? Could he? Should he? she asked."
Rhetorical Question Effectiveness

"The effectiveness of rhetorical questions in argument comes from their dramatic quality. They suggest dialogue, especially when the speaker both asks and answers them himself, as if he were playing two parts on the stage. They are not always impassioned; they may be mildly ironical or merely argumentative: but they are always to some extent dramatic, and, if used to excess, they tend to give ones style a theatrical air."

"Rhetorical questioning isa fairly conscious technique adopted by a speaker for deliberate ends, and it is used infrequently, proportional to the length of the dialogue, oration, or conversation."
Rhetorical Question