Stevie: What's the... nuclear option?
Reese: Evidence of the worst thing we ever did.
[Reese, Malcolm and Dewey each grab a key from a chain around their neck and unlock the three padlocks on the box]
Malcolm: This is what made sure that no matter how bad we screwed each other over, there was always a limit. None of us could push anyone to a point where they had nothing to lose. Or they'd just pull the trigger and destroy all of us.
Stevie: How bad... can it be?
Malcolm: One time we made Mom think she had cancer. But we had a really good reason. [flashback to Lois and Hal sobbing in the kitchen:]
Lois: Stop saying it'll be all right! You'll never be able to raise them alone!
Young Malcolm: Mom, I know this is a bad time, but can you sign these report cards?
[present:]
Malcolm: It's not like we're proud of it. I mean, we were proud of it, but we're not proud of it now. The point is, we can get rid of it.