There's something you should know about Peter. He's not quite right, if you know what I mean. See, his mother is very busy because she works all day at the diner and all night at the local pub. Peter's dad is around all the time, but he talks a little funny and has trouble walking sometimes. He hits Peter because he doesn't do very well in school, but Peter knows that Daddy only hits him because he loves him and wants him to do better. The only thing in life that makes Peter happy is his cat, Mr. Whiskers. Peter's older brother, Matthew, is very smart. He makes straight As in school, and he's a decent baseball player, but it's not quite enough. See, Matthew gets bored very easily, and his imagination doesn't always lead him to the nicest places. Tormenting Peter is one of his favorite things to do, and one hot July day, that's just what he was doing.
MATTHEW: Guess what I've got in this garbage bag, little brother.
PETER: What? You're not going to dump icky garbage on me again, are you?
MATTHEW: No. Just guess. It'll be fun.
The bag starts to shake a little bit, and MATTHEW tightens his grip. The contents are obviously alive.
PETER: Is it a frog?
MATTHEW: No, guess again. If you don't get it after three tries I'll just show you.
A light tinkling sound can be heard to be coming from the bag. Whatever is inside it is struggling hard to escape.
PETER: It looks like it wants out. I don't want to play this game anymore, Matt. Let it out.
MATTHEW: Guess what it is, and I'll let it out.
The bag softly meows, then goes limp.
PETER: That sounded like...
MATTHEW: No. No, Pete.
PETER: The bell, the meow, the size. IT'S MR. WHISKERS! LET HIM OUT. LET THE CAT OUT OF THE BAG!
MATTHEW dumps the now-dead cat out onto the sidwalk. After verifying that the cat is, indeed, MR. WHISKERS, PETER hits MATTHEW over the head with a large rock. He is bleeding and will undoubtedly die soon. PETER picks up MR. WHISKERS and strokes his fur.
PETER: (crying) Mr. Whishers? Mr. Whiskers, are you okay? (beat) How could you do this to Mr. Whishers, Matt? What did he ever do to you? What did I ever do to you?
PETER: (crying) Why couldn't you just let the cat out of the bag? None of this would have happened if you'd just let the cat out of the bag.
Oh yeah, "let the cat out of the bag" is also an expression meaning "to ruin the surprise" or "to disclose a secret." I guess some old sayings aren't meant to be taken literally.