Background: I work as an engineer with someone who is an "engineer" by trade only (not by training or formal education). Un/fortunately, before asking difficult questions, he researches information, or just hears about it online and tries to muddle his way to an understanding of it. He recently heard a discussion on time dilation and how it takes an object a longer amount of time to travel 4 light years than light. Quandary: I am unable to explain in lay-terms why this time dilation happens. This, of course, means that I do not a full understanding of the matter. I would like to be able to explain, and would like help. I am not a physicist, but I will try my best. Simply put, he wants to know if an object (say a spaceship with a man inside), if traveling at the speed of light and a strong beam of light (perhaps a very strong laser) were aimed at the moon, would they arrive at the moon at the same time? He thinks that the man in the ship could look out the window and see little particles of light "growing" alongside the ship (even with the ship as well). It is my understanding that as the object's velocity increases, so would it's mass; and that the object would experience contraction. How do I explain that it is not "as the acceleration increases" that the objects mass increases to a lay person? Also, further help in explaining this (to me) would be greatly appreciated, so that I may understand fully, and be able to explain the answer to his question. Thanks much!