as speeds get closer to the speed of light, then the changes in mass become impossible not to notice. the basic experience we all share in daily life is that the harder we push something like a car the faster we can get it going. but when something we are pushing is already going at some large part of the speed of light we find that it keeps gaining mass, so it gets harder and harder to get it going faster. it is impossible to make any mass go at the speed of light because to do so would take infinite energy.