User blog comment:DarkSkullPirates/He's Just Bones, So Where Does It Go?/@comment-4904935-20120704041446

It has to do with Brook's DFP ability. As long as the soul in his body, he is alive like normal human being, feeling pain, getting tired etc. This is why he can still eat, speak, cry...and even poop, even though he doesn't have the important organs to do any of those things.