Death.

[antigrav]

Why is it considered necessary to nail down the lid of a coffin?

[ugly_bob]

just in case they turn into Zombies

[WoWo]

because the last thing you want to happen is for the body to be falling out all over the place if it was dropped or held at an angle.

Seeing as whats in side isn't going to need to get out easily there isn't any point putting any thing elaborate like locks etc to keep the lid on.

[Jay_Jay]

Many lids of caskets nowadays are screwed down, rather than nailed shut, with something which resembles a key. It may be something to do with the final act of committing the body to its final resting-place.

Sometimes the body is viewed by the mourners whilst in an "open casket", then the cover is screwed shut.

[DotH]

centuries ago when grave yards were not walled in and pigs could roam freely over the common land, they would dig the buried corpses up and eat them, it was then discovered, (by trial and error) that a pig could not get out of a 6ft hole, therefore graves were dug 6ft deep and bodies were buried in caskets and nailed down just in case the pig actually risked it and junped in anyway.

[SaxyJag]

One of my husband's female ancestors is famous in the family for having left instructions for her coffin to be lead lined, so desperate was she not to become worm food.

That's a silly idea, really, because you know, lead's really poisonous - you can die from lead poisoning. Still, I don't expect they knew that back then, did they. ;-)

[Octavius]

It is to make sure that if the person inside is not dead, that they soon will be and they won't be able to jump out. Any knocking would just be seen as the head bouncing up and down under the lid.

It was also to perpetuate the saying, 'thats the final nail in the coffin' as in - he won't be getting up/out again.

[andy-hughes]

I heard it was to deter grave robbers in the nineteenth century. As medical science took on more radical exploration, the need for fresh corpes to experiment with meant a lucrative living for some of the less savoury individuals with a shovel and good muscles. The act of nailing down the lid of a coffin presented just one more obstacle to the robbers, who may have abandonded the coprse and looked for some easier prey elsewhere in the graveyard.