Software systems situated in network environment may experience performance degradation, availability decrease and even crash during long time running, which is called software aging. This phenomenon has been studied for more than 15 years, but most of the literatures studied software as a black box, none of them uncovered the fundamental and widely accepted mechanism of software aging as far as we know. Through analyzing the characteristics between biological aging and software aging, we find some interesting common points and bridge the gap between these two seemingly unrelated phenomena. The free radical aging theory in biological studies is also applicative to explore the mechanism and model of software aging. This paper finds an equivalent concept named `software free radical' in software aging to free radical in biological aging. In our study, the accumulation of `software free radical' is a root cause of software aging. Using the free radical modeling methodology in biological aging, we give a model for describing the kinetic of software aging based on feedback loops. Although this paper doesn't give enough theoretical proof of the modeling method, the practical results show that the feedback loop model can describe the kinetic of software aging precisely. To further validate the aging mechanism, we propose several software rejuvenation strategies focusing on cleaning the `software free radical'. The results show that software aging can be mitigated effectively by strengthening negative feedback loop or weakening positive feedback loop. This paper is the first try to answer the question `How software ages' through interdisciplinary studies. Leveraging the conclusions in this paper, people can design better software systems or keep their systems at a high performance level during long time running.