Theory:

This poem is about a little girl who wants to make breakfast for her grandmother. She asks her grandmother what she wants, and the grandmother says that she wants to eat Johnnycake for breakfast. Making a Johnnycake requires flour, butter, and sugar, but Sarah does not have any flour left. So, she tries to get help and make the cake.
 
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She asks the grocer, farmer, miller, ploughman, even the sun to grow the corn for the flour, but the sun is also helpless as no one has sown the seed. Finally, when the ploughman ploughs the land, the farmer sows the seed, the sun helps the cornfield grow, and the miller grinds flour for her. Sarah gets the ingredients and mixes it in one minute and bakes the cake in two minutes. She gives it to her grandmother, and after all the hard work, her grandmother wants another one.
Reference:
Image courtesy: https://commons.wikimedia.org/wiki/File:Johnnycake,_West_Indies_01.jpg