Summary The word mass has been used at least since the 17th Century. However, after 400 years, the mathematicians don't have the slightest clue what mass is. Yet they claim that this unidentified 'thing' increases when an object travels at close to the speed of light. Relativists argue that their theory has been proven beyond a shadow of a doubt and you can no longer challenge it. And then again, we remind them that they have no idea what they're talking about. When we investigate this enigmatic word, we discover that most people believe that mass alludes to the quantity of matter of a substance. This is patently wrong. During Special Relativity's mass increase, relativists can't seem to make up their minds what it is that actually increases, whether the amount of substance or just its speed. Where practically everyone agrees is that what really increases is the energy content of the traveling object. It turns out, however, that despite that the word energy has been around longer than mass, no one has any idea what energy is either. We end up explaining the increase in mass which we know not what it is, in terms of an increase in energy, which we also know not what it is. Yet relativists insist that their theory is beyond challenge.