Does CI persists?

[eluser]mlakhara[/eluser]
This may be just a dumb question(even for beginners), but I was struck at this confusion two days ago and have not been able to figure it out through my internal conscience.
Does CI framework persists between two requests by the client?
for example if I set a static variable's value during in one model and try to access that value after some time, would it be fundamentally correct for me to expact that value to be set that way?