Why does Allah keep on changing his challenge.. is it out of fear..?

Why do I answer your question. Not for you but for others to know. Since Allah has put seal on your heart I cant make you understand anything. Allah has created the mankind and He knows what goes on in their minds and hearts and how do they think. Allah knows that mankind doubt to His revealation (its because of being victim of satan) so He answers it. Like for example you yourself is one of the example. If Quran is not from Allah and is just a man made then why cant u do it. What one man can do the other can do as well. So take up this challenge. And bring an example of the Quran. The Allah is not talking about just words. The words contain a sense. That sense u cannot produce. Try it...................