"If the world were to blow itself up, the last audible voice would be that of an expert saying it can’t be done." – Peter Ustinov

It's interesting stuff these limits we put on ourselves and others by arrogantly declaring "it can't be done". I was once challenged to eat a loaf of bread and did so with ease. The prize was a couple of Big Macs which I cleaned up no problem right afterwards. The strange thing is we place all these constraints on ourselves. That is, when we are born we are pure, but quickly tainted by our parents and others to make sure we have little chance in truly interacting as humans to our fullest potential. Did you know that up to about 6 months, if a baby is placed in water it will come up for a breath naturally and then of its own free will, it dives back down again in the water as a natural reflex. By choice. There is not much of that going about later on in life though. Watching the athletes in the Olympics breaking records it is so cool to see that the boundaries are overcome really quickly. I mean from one four year period (not a long time in evolution of man at all) to another the performances get better and better. It does take a huge effort to pass the existing marks set by others. Far too much energy as much of it is spent on a coach trying to psyche his charge up to believe they can actually take on the records. Then you get a guy like Michael Johnson or Phelps (he eats about a dozen eggs in a sitting ...they would have a good home with the loaf I mentioned earlier) who are specimens not too bothered about other peoples performances and have supreme confidence in themselves. They seem to cruise past the records. The African long distance runners are not just breaking the seemingly incredible feats already set up by their countrymen in previous years, they are doing so at the ages of 18 years and 20 years old. It does not make sense to the masses watching, especially those that know some simple maths. Where will the records stop in the question you see ... there is simply no answer that will come out in the form of a year, date or time now is there? So when will they stop? How can they though and why must they. Because eventually they will reach zero you answer, and you can't run the 100m in 0 seconds can you.
0 is just a number, a barrier at the end of a scale created by our good selves. I reckon that Jamaican Bolt characters great great grandchild - Lightning, will still be knocking that time down steadily in 60 years time and we'll still be saying it can't be done.