rhetorical question are essentially questions that dont need to be answered. like if a teacher asks you a question where he/she knows the answer to, or he/she knows that you know the answer but does not ask you to answer

A 'rhetorical question' as described above is a rhetorical device (often called a 'figure of speech') called Hypophora (they all have fancy Latin or Greek names, unfortunately). There are dozens of different devices.

A good example would be 'Ask not what your country can do for you - ask what you can do for your country.' This is a device called Chiasmus, where the words in a clause or phrase are rversed in the next, i.e. 'country ... you' becomes 'you ... country.'

The purpose of Anaphora is drive a point home into the listener's (or reader's) brain through repetition.

It repeats the same word or words at the beginning of several successive phrases or sentences (as opposed to Antistrophe, which does it at the end).

Repeating it twice might be overlooked by the audience or just regarded as sloppy editing, so most users of Anaphora will repeat it at least 3 times (though some can repeat it a lot more, e.g. Churchill's use of 'We shall fight ...' and Martin Luther King's 'I believe ...'

It's possibly the most commonly used rhetorical device and is a favorite of President Obama's. Some examples of his would be:

"That means investments to create new jobs. That means resisting the walls of protectionism that stand in the way of growth. That means a change in our financial system, with new rules to prevent abuse and future crisis."

"For us, they packed up their few worldly possessions and traveled across oceans in search of a new life. For us, they toiled in sweatshops and settled the West; endured the lash of the whip and ploughed the hard earth. For us, theyfought and died, in places like Concord and Gettysburg; Normandy and Khe Sahn."

"This is the price and the promise of citizenship. This is the source of our confidence - the knowledge that God calls on us to shape an uncertain destiny. This is themeaning of our liberty and our creed."