Dont mention 'engine failures'!
The punter will often mention them first - "what happens if the engine stops?"
So I ask them what makes the thing fly ... with a variable amount of prompting they eventually get to "the wings" (on bad days I have to ask "how do gliders fly then" to get there). They then understand that if the engine stops the aircraft will continue to fly and they stop worrying.
If they're really keen I'll show them that lack of power dosn't mean falling out of the sky by pulling the throttle to idle whilst in the cruise.
Seriously though, I do include in the briefing that if we have to land in a field I'll ask them to open the door whilst we're still in the air and I really mean it - that won't be the time for them to start questioning whether I really said that and really meant it.