A number of people here have indicated “donating their body to science,” and that might include donating their body to a medical school to be used for practice and instruction for medical students. Medical schools usually take the entire physical body. Most medical schools will embalm the body and use it repeatedly to instruct students until it starts to decay. Although the skeleton, including the skull, etc., are usually kept for an indefinite period of time and used for instruction.
It was also mentioned here about being buried alive; rest assured if you die in the U.S. and receive an autopsy, your body will be dead before they bury it. If you do not receive an autopsy, that is another story. I have seen people wake up in the morgue, before an autopsy, after a doctor had pronounced them dead. There are also stories of people being buried alive, or placed in a cremation furnace, and then waking up before the fire is turned on.
In the U.S., there is “clinical death “and “brain death.” Clinical death is the cessation of blood circulation and breathing, while brain death is the cessation, or interruption, of brain activity. Biological death occurs when there is no more activity in the brain and the heart, but a person can still be kept alive on machines if the family so desires, or it is indicated in their previously submitted medical directive or Last-Will & Testament, etc. Still, in my opinion, there is a blurred line between what constitutes life and what constitutes death. There are people who are brain dead but they are still alive because their heart and lungs are still functioning on their own without the help of a machine.
These are things that people indicate in a medical directive, i.e. if you are brain dead do you still want to be kept alive, or if your body can not draw breath and pump blood on its own, do you want to be kept alive on machines, etc. There are lots of such scenarios. I have worked with patients who were in a coma for a very long time, months or even years, and they are still alive, although usually bed-ridden and kept alive by machines. I think it is important to not only indicate how you want to dispose of your body at the time of death, but also do you want to be kept alive if you are in a coma, have no brain activity, etc. End of life decisions that others, loved ones, your physician, attorney, etc., may be helpful for them to know.
"Life is like a box of chocolates." Forrest Gump