@peter jeffrey cobb,
Peter I am not a physic teacher however the reason nothing can reach the speed of light can have any rest mass is that as you approach light speed the mass increased to the point that all the energy in the universe is not enough to increase it speed further. All the energy at your command is being used to increased the mass of the object not the speed.
Put another way mass is not a constant for an object it depend on it speed.
Even a baseball you had thrown had a very very very very small increase of mass over the ball in your hands.
At everyday speed/velocity this is such a small change that no one can measure it however as your approach a large percents of the speed of light that change in a big way.
Let see the equation is that for any given velocity the mass increase over the rest mass as follow.
Mass (at a given V}=mass(rest)/(1-V^2/C^2)^1/2
The equation is such that at C the velocity of light the rest mass is increased without limit and all the energy you can pump in then in is taken to increased the mass not the speed.