The Apple You Kept Away

It's like what happens to a cut apple, you know. When it's cut or part-eaten and just left out in the open and it goes all dark and spoilt. It shrivels and seems to die, as if it was alive while feeding you. Strange, innit? You leave it unattended for a while, and when you're back it's almost wounded, of no use but to be thrown out. Why would one do that,do you know?