The aim of this article is to inform the teachers and students of secondary schools about the intricacies of internal computer workings with real numbers. It is surprising how many people think they found a bug in Microsoft Excel that ought to be promptly fixed by the developers. Experience shows, however, that it is not enough to tell these people they should read up on coding the real numbers in computers. In this article, alongside simple mathematics we have also chosen to represent the “computer numbers” on real axis which should help towards a better understanding.