A Physics Quiz of a different type

[DonRotolo]

Last night, the neighbors came over for a visit, and (somehow) we got on the topic of Relativity. One neighbor, a non-science kind of guy (but very intelligent) wanted to know why you can't go faster than the speed of light.

My response was that at one end of E=MC^2 you have pure mass, and at the other end you have pure energy. You can't go past the endpoints. As you go closer to C, M decreases and E increases, since C is decreasing*

*That is, since C is relative to a fixed point, as you hit, say, 0.75 C, there is only 0.25 C "left", so C is actually smaller.

OK, he accepted that as reasonable , but in my mind, that's not right.

So, in one or two sentences, why is there a limit to the speed of light?

Thanks.