Many people believe that a person is the brain or some part of the brain. You may be one of them. If so, the following should boggle your mind:

Recent studies on the turnover of the molecular population within a give nerve cell have indicated that … their macromolecular contingent is renewed about ten thousand times in a lifetime.*

In other words, the matter making up each brain cell is completely renewed every three days. Your brain — that mass of matter which is contained in your skull today — is not the same brain that was in your skull last week.

~ Chris Butler (Jagad Guru Siddhaswarupananda Paramahamsa)
Science of Identity Foundation

* _{Paul Weiss, “The Living System: Determinism Stratified,” in Arthur Koestler and J.R. Smythies, eds., Beyond Reductionism (London: Hutchinson, 1969), p. 13.}