Hmm. That's a potential explanation. I think I like that.

Hey, just floating around the site and reading old stuff.. saw this and thought I'd comment...

Looking at how the grenade seen in the deleted scene of the movie works (when Inara and Mal are fleeing the training house), the top of the grenade is twisted to unlock it and allow it to 'pop' up, which then activates a counter that gives you time to throw before it explodes.

here's a video of a hero prop version of the grenade as an example: http://s100.photobucket.com/albums/m39/Whitefall/?action=view¤t=bombvid.mp4

Considering that, I could see how a griswald could be a small grenade that has the same sort of mechanism... imagine the grenade is compressed and armed (but not locked) then inserted into the apple (how they did that exactly still eludes me, perhaps they used a small coring device, inserted the grenade and then replaced the cored section??) When someone bites into the apple and the greande is no longer under pressure and allowed to 'pop', it explodes...

That'd be my take on how they wouldn't go off when landing on the ground, but would once bitten into...

Actually, thinking about it further, even if you layer it backwards the charge is still most likely to be set off by the initial compressive force. the only difference is instead of moving the screen towards the circuit, you've move the circuit towards the screen.

And probably die. Because you'd be biting into an electric circuit.

http://www.freepatentsonline.com/4233523.html

See? You put a load (or compressive force) on the outer screen, which causes activation.

Now, it's possible there are other arrangements or that you might layer everything backwards, but when you're talking human interactions with an object you're more likely to encounter compressive force. It's just what makes sense. Biting into the apple is first a compressive force, removing the apple bits is decompressive, but you're still going to have the compressive first and it's still the most likely cause of the explosive activation.

Hmm... That isn't usually how pressure sensitive gizmos work. It's usually based off compressive pressure, positive force over an area (biting into the apple, hitting the ground) as opposed to negative force.

Maybe the three guys were so hungry they didn't notice the other apples blew up...

Fridge logic 2: They were sealed inside the apples then activated, and only went off when the unfortunate souls bit into them, thus *reducing* the pressure inside.

Could be ...