Lucy approaches Charlie Brown on the lane as he rides his tricycle. She says, "I'll tell you what, Charlie Brown..."
She runs along beside him and continues, "I've got a baby brother at home..I'll trade you him for your tricycle!"
He rides off and asks, "Absolutely not! What would good would a baby brother do me?" She replies, "I knew you'd say that, and I don't blame you at all..."
She concludes, "He's no good to me, either!"
Linus!
Wonder if Schulz had a plan for introducing Linus this way, or made the joke and then later decided he might as well introduce the character.