you want to place the entire grenade inside of an empty can, just tight enough to hold the spoon in place. then you remove the pin, and you secure the can, so the string pulls the grenade out of the can, and the spoon kicks itself off.
actually for the least resistance to the tripwire, you want to pull the can off the grenade
if you're particularly diabolical, you'll replace the grenade fuse with a smoke grenade fuse which has no delay
Intresting. With a no delay fuse, you'd have to have the can as close to the opening as possible, otherwise it'll go off without direct line of sight to the victim pranked and they might survive not fully appreciate the joke.
I guess the effectiveness and line of sight also depends on which way the door opens, standards on this differ wildly between countries. If the grenade explodes as soon as it exits the can, and the can is more than a grenade-length away from the door, much of the direct detonation shockwave and shrapnel would be blocked by the wall being between the explosion and the pranked person, especially if the door has to be pulled open. If the door is pushed open, chances are that at least the opening hand and arm is exposed.
It also depends on the material and thickness of the wall, of course. I have personally been about three feet from a hand grenade going off, with no I'll effects other than some dirt falling on me. There was of course a bit of reinforced concrete between me and said grenade. It was also outdoors which obviously makes a huge difference on experienced pressure.
Hitler, for example, survived a pretty big indoors explosion just a few yards away, only because he was mostly shielded by a table. Some windows may have been open which affects lethality more than one might assume. Didn't Mythbusters test exactly this?
It is also quite possible I've given this a lot more thought than I should've. When you know a lot about a subject, you want it to be correct. This is not such a subject.
the setup I was describing, I've only ever actually seen used outdoors, along suspected lines of travel
