There are two versions of this premise. There's the one where one of the characters directly compares CB's head to a ball (usually concluded with a chase), and there's instances such as this one where a character indirectly remarks that Charlie Brown's head is round like a ball.
The very first joke about the size and shape of CB's head was of the second type, when the kid was in the water and Patty and Violet wondered why a beachball had floated out so far.
This is far from the last joke about the poor boy's head.
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