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They wanted to see the Scandinavian boys visiting America for once and get a taste of American culture, Peanut Butter being of special interest. We do have Peanut Butter up here in the North now like most American stuff, but for the sake of the comic lets say this was the first time they encountered it.
I have no idea how many people will get the last part, but it's a joke on the old American show "Mister Ed" from the 60's about a horse that could talk. When asked how they got the horse to move it's lips when "talking" the people behind the show claimed it was done by putting peanut butter on it's gums (they later told they had trained it to do so, but didn't want to disappoint the children who watched the show).
I honestly have no idea why I know about that show in the first place.