Yes, provided you didn't release the safety trigger. Assuming you're using a grenade that has a 2nd safety trigger to begin with. If it's those Civil War grenades, you're fucked.
Nope, the pin is kinda like the thing that would hold a hatch door closed, if it's removed it will release a powder that reacts with the powder already inside, creating a timed explosion
Not necessarily, in a fragmenting grenade like you see in the picture above, if you pull the pin, it can be put back in place, however if you remove the spoon, the handle looking thing, then it releases the powder to ignite.
Yeah, it's like a fire extinguisher. You can pull the safety pin and stick it back in all you want, but once you pull the lever and slosh all the stuff everywhere then that's the point of no return.
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