As with the rest of her family, Elizabeth Eliza is book-smart but somewhat lacking in the common-sense department. This means that she often gets into difficulties that wouldn't have arisen had she been a little more practical-minded.
On one such occasion, she comes up with what seems like a good idea: to go to an old woman selling herbs to find something to put into Mrs. Peterkin's tea to make it taste better. The scatter-brained Mrs. Peterkin put salt in her tea, so it tastes disgusting. Elizabeth Eliza thinks that if they stir in enough herbs, then that will somehow take away the foul taste of salty tea.
In the event, the plan doesn't work. No matter how many herbs the old lady stirs into Mrs. Peterkin's tea, it doesn't taste any better. It's then that Elizabeth Eliza comes up with a genuinely smart idea: why don't they go to the wise lady from Philadelphia? She'll know what to do.
As it turns out, Elizabeth Eliza is right. The lady from Philadelphia, the paragon of common sense, comes up with the novel idea of making a fresh cup of tea. "Why didn't we think of that?" says Elizabeth Eliza.