Chocolate can kill a dog

Why does a dog eaten chocolates can cause of its death? I wonder why? Yeah, I know there is a scientific explanation but my science teachers didn't discuss it on us way back on school days. But a puppy on the picture have died because of heat, a high temperature. We never know that it will happen, we have no idea. So if you want to raise a puppy, be sure that you will really take care of it like taking care of yourself.