What is time? And space? How are these two apparently opposed elements indissolubly linked? This book starts from man's intrinsic need to ask himself questions and elaborate answers, and tries to analyze, through a simple and intuitive physical-mathematical language, some aspects of Einstein's Special and General Relativity, accompanied by equally clear and immediately understandable images. An interesting discussion of simultaneity and causality is presented together with the relativistic accelerated version of the original twin paradox.