What is the miracle in the Muqatta'at Letters in the Qur'an?

Another miracle of the Qur'an can be found in the mysterious usage of the so-called Muqatta'at letters that occur in certain chapters. The choice of Muhammad (SAW) as the Messenger of Allah was miraculous because he could neither read or write. He had no knowledge of the skills of literature, yet he preached to mankind in a language that was unique in terms of its clarity, intelligibility and its inimitability. All these factors were clear indications to the Our'an's divine origin. An unlettered person can utter words and reproduce sentences, but cannot construct new ones. Many of the passages revealed to Muhammad (SAW) begin with the letters of the Arabic alphabet, such as alif, lam, mim as a further challenge to the sceptics. Despite being unlettered, Muhammad (SAW) was able to pronounce those letters in which he had no education and which he had never previously heard spoken. If the Qur'an were of Muhammad's (SAW) own making, the pronunciation of the letters of the alphabet, which he had not learnt, would have been beyond his ability, for only a person who had learnt how to read and write could identify these letters and utter them correctly. - Sharawi, Miracles of the Quran