The White Queen mentions that someone said they would come in because they were searching for a hippopotamus. However, that morning, there was no such creature in the house, which seems to be part of a whimsical or imaginary scenario. Alice is curious and asks whether hippopotamuses are usually found there, showing her astonishment at the strange conversation.
The Queen responds humorously that hippopotamuses only appear in the house on Thursdays, highlighting the fantastical and nonsensical tone typical of Lewis Carroll's writing. This exchange illustrates the playful and surreal world Alice is exploring, filled with odd creatures and illogical ideas.