Nice question. You're assuming that if the light is travelling at speed
c away from you, as measured by you, and if you're going at speed 0.99c
in the same direction away from some other guy, according to him, then
the light must be going at 1.99c away from him, according to him. That
is what common sense says (actually, this is what's called a "Galilean
transformation", and it is useful and convenient, but only for speeds
much less than the speed of light). If you try to write out why you
believe it must be so, you will find that you make two assumptions:
1. That the time interval between two events (say light leaving a
bulb and hitting a wall) is the same according to you and the other
2. That the distances between things (say a bulb and a wall) are the same according to you and the other guy.
It turns out that both these assumptions are false. Einstein's
Special Relativity gives a nice description of how these assumptions
break down, and what the rules are for replacing them.
(republished on 07/23/06)