How smart are you

[Tread]

If someone is still brooding about it, here are the remaining answers:

3. Grave
It?s an exception that you dig your own grave (not proverbial).
A coffin is not as limited in use as a grave, but to put a corpse in it would only be useful if you want the corpse back, or more common to transport it (witch would be a use). If you visit the grave you don?t use it.
If someone knows he uses it, something went terrible wrong.

4. Fire
I found the life aspect interesting:
it has a metabolism,
it creates a balance between intake and excreta to keep the flame stabile,
it response to stimuli,
the ability to change over a period of time in response to the environment (as embers, smolder),
it moves, it grows, it reproduces itself (over sparks),
and it dies.