Explain Like I'm Five

Ocean mine.

Many sea mines of that era are actually activated by disturbances in magnetism! They're pretty good about not going off unless something large and metal rolls up, since that's their purpose, although anything really old is suspect of not working as designed. That's why when somebody digs up (or just kind of has) an old bomb they detonate it on site instead of trying to move it.

WWII also saw acoustic mines used. Contact mines were and are around too, and I would guess if you don't crush a trigger the same answer about only going off when conditions are met applies.

Modern low-sensitivity explosives can survive an actual plane crash without going off, and will burn instead.

helmet (why not just move the whole sentence into the text, OP?)

SSTF covered everything from here on pretty much perfectly.

On the off chance your helmet does successfully take that kind of punishment, and the blast wave is absorbed enough to not hurt the soldier, there still will be a bunch of hot gases escaping in every direction which isn't great. Better than no helmet, though - the trick is just noticing the grenade, stopping, removing your helmet and gently placing it over top before it blows up, which is why that move isn't actually a thing.