Throughout the book Goblet of Fire Harry uses the word "Er" whenever he doesn't know the answer to a question posed by his professors or friends. He says "Er" to Snape, Professor Mcgonagall, Dumbledore, Hermione, Ron and others. While reading through for the second time, I found this extremely odd . . . but in the final task when the sphinx has a riddle for Harry he says:
First think of the person who lives in disguise,
Who deals in secrets and tells naught but lies.
Next, tell me what's always the last thing to mend,
The middle of middle and end of the end?
And finally give me the sound often heard,
During the search for a hard-to-find word.
Now string them together and answer me this,
Which creature would you be unwilling to kiss?
Harry correctly guess the word "Spy" and "Er", to get the answer "Spider."
— Quora user Prabhu Rascal