Relativity, the Special Theory, explained to Children (from 7 to 107   years old)
  The author thinks that the main ideas or Relativity Theory can be explained to children (around the age of 15 or 16) without complicated calculations, by using very simple arguments of affine geometry. The proposed approach is presented as a conversation between the author and one of his grand-children. Limited here to the Special Theory, it will be extended to the General Theory elsewhere, as sketched in conclusion. 
Charles-Michel Marle (IMJ)
physics.pop-ph