Just asking. Brook came "back to life". But is there a way to actually kill him (again)?
EDIT: What I find really strange: Devil Fruit users lose their power in water, but the power of the Yomi Yomi fruit is to bring the one who ate it back to live and - so it seems - keep him alive. So in theory if Devil Fruits don't work under water shouldn't Brook die instantly? But he doesn't... O_ô