Seriously, how do you keep a straight face?

Dinner is in the oven, Liza's artwork is drying on the easel downstairs, and the whole family is sitting on the floor playing Uno.  The cat wanders over and plunks down on my lap, where I notice that all is not right.

"How did Zach get paint on his head?  Did we leave anything out where he could rub against it?"
(quiet sobs start)
I pause for a second in disbelief, then ask, "Liza, did you paint the cat?"
(wailing begins)
"I didn't mean to!  He was just right there!  Arghlfarblepthththt!"
(Jason and I can't look at each other, or we're totally going to bust out laughing, because she is soooo repentant)
"You painted the cat.  You actually painted the cat?  Why would you do that?  You know that's wrong, don't you?"
"I don't know!  But he was there!  Arghflarghpththth!"
(Seriously, we can't even look in each other's direction, or we're going to lose our "stern parent" front)
"Well, that was wrong.  We can't paint the cat - do you understand why?  What's going to happen when he cleans himself?"
"He's going to puke all over the floor again."
(sniffles and much moping)
"That's right.  You did something wrong, and we're going to have to punish you for that, but I'm glad you told us the truth about it.  You have to draw six extra cards and skip a turn in the game.  And I'd better not see the cat with a purple forehead ever again, do you understand?"
"Yes, mama.  Oh, look!  I got a blue one!"