In the short story Motive v. Opportunity, Miss Marple used as a parallel the actions of Tommy Symmonds, a boy in St. Mary Mead who was mischievous but sometimes very amusing, "[o]ne of those children with innocent childlike faces who are always up to some mischief or other". He once played a prank by asking his sunday school teacher Miss Durston whether one should say "yolk of eggs is white" or "yolk of eggs are white" Miss Durston explained that the correct format would be “yolks of eggs are white" or "yolk of egg is white”. Whereupon Tommy responded that yolk of egg should be yellow.
Miss Marple likened Tommy's prank to Mr Petherick's story because it had a catch in it.