Why does our brain trigger us to itch a mosquito bite? It doesn't seem to accomplish anything useful...Nothing useful at all. We have two sets of nerves in our bodies, one for sensing big impacts on the body and one for sensing gentle interactions with the surroundings. This is why rubbing an injury decreases the pain experienced -- the nerves for the big stuff are sending their signal to the brain causing the pain, but the light rubbing fires up the other nerves telling your brain that there is something gentler going on. This forces your brain to split its attention and thereby give less focus to the pain signal. In the same way, scratching an itch can not only dislodge the irritant causing the itch, but triggers the big contact nerves and keep the brain from focusing on the sensation of the light touch nerves that causes the itch sensation.
When a mosquito bites you, it injects a chemical to stop clotting so it can take the blood it seeks. As a result, your body sends histamines to remedy the situation and this in turn causes a swelling in the blood vessels. This stretches the skin ever so slightly and that disruption from normal triggers the signal from the gentle touch nerves which the brain interprets as an itch. The itch is just a by-product of your body trying to heal itself from the small injury inflicted by the mosquito.
Each year the world's fastest man seems to break his own record (by 100ths of a second). Since no one could conceivably run a 2 second 100 m dash. What is the maximum speed possible for the human body to run? There must be a physical limit that can be reached.Like yesterday, we cannot put a number on it, but yes, there will be a limit. Improvement in training techniques and athlete size may always make a record breaking run possible, but like Zeno's racetrack, there's a convergence to a lower limit. You see this in baseball where pitchers throw no harder now than at any time in history. Nolan Ryan threw 100 miles per hour and while you'll get someone who can crack 103 or 104 from time to time, you don't see major jumps. The human body works the way it does and this does put constraints on how fast we can run or throw.