In high school physics we've always been told that test will try to trick you. They'll ask if a 10kg person goes from the earth to the moon how will their mass change. And the answer is always it doesn't. Mass doesn't change, mass doesn't change, mass doesn't change. It's been hammered into our brains. But it's a lie. So the speed of light in a vacuum is 300,000 km/s. This is the fastest speed any object in the universe can travel at. So what happens if you try to accelerate an object going the speed of light? Well picture this: a rocket accelerate to the speed of light, but the thrusters are still pushing on the rocket. You might be tempted to say that the frictional force balances with the thrust of the rocket, so there's no net force. But then how would the rocket have accelerated to the speed of light? There must be a net force. Given that there is a net force, work is being done on the rocket. Therefore, there is a change in kinetic energy, but velocity isn't increasing. That means the other component of kinetic energy must be increasing: mass. In most cases mass is a constant, but when energy cannot be transferred into speed any longer, it has to be transferred into mass instead.