From the time the Quran was revealed, until this day, there has always been a huge number of Muslims who have memorized all of the Quran, letter by letter. Some of them have even been able to memorize all of the Quran by the age of ten. Not one letter of the Quran has been changed over the centuries. This also proves that Muhammad is truly a prophet sent by God. It is beyond reason that anyone fourteen hundred years ago would have known these facts discovered or proven only recently with advanced equipment and sophisticated scientific methods.
Some examples follow.