Why is this the answer?

The answer is short because this riddle is a super clever trick that plays with words and how we spell them! It is one of those fun puzzles that makes you think one way when the answer is actually a surprise about letters. Let us look at the first part of the riddle: "What five-letter word…" The word "short" has exactly five letters: S H O R T. We can count them to be sure. That is our starting word for the puzzle. Now, let us look at the trickiest part: "…becomes shorter when you add two more letters?" The riddle is not asking you to make the idea of "short" become less long. It is asking you to add two *actual letters* to the spelling of the word S H O R T. What two letters should we add to make the riddle work? We add the letters E and R. When you add E and R to S H O R T, the new word you get is S H O R T E R. The new word is spelled out as "shorter." The word "short" magically *becomes* the word "shorter" just by adding two letters. Now, here is the secret to the riddle. The new word, "shorter," is actually seven letters long, which is *more* letters than the original five. But the *meaning* of the new word is that something is less long in size or length. The word itself tells you it has become "shorter," even though you used more letters to write it down! The riddle works because the word you create describes the final result of the action. You have to think about the spelling and the meaning at the same time to solve this great word puzzle.