I have just recently began to understand relativity, right now I get time dilation and lenght contraction but I do not get relativistic mass. The book I am reading does not defy it very well (or perhaps I should read the chapter again) but it gave it as the explanation of why objects with mass cannot reach the speed of light. The book says that as the object increases its speed its mass increases making it require more energy to speed up, ultimatly ending in it requiring an infinite amount of energy to raise its speed. As of now I only have a ninth grade education and according to it mass cannot be created or destroyed, however the book implies that mass is being created. I thought it may be an illusion, but the book claims relativistic effects are real and arent illusions. Right now the best explanation I can think of is the kinetic energy of the moving object is being converted into mass at such high speeds(giving rise to high energies required for mass conversion). I would really appreciate the true definition of relativistic mass, so please help me out.