Why is this the answer?

The answer is past because this wonderful riddle is playing a clever trick with words. It makes you think about climbing a big hill, but it is really describing how we look at time. Imagine you are on a long walk and you climb a very tall hill. The riddle says, "Half-way up the hill, I see you at last, lying beneath me." When you are up high, the whole area you walked through to get there is "lying beneath" you. That journey you already finished is like your *past*. It is something you have moved beyond and are now looking back at. The riddle describes what you see below as "A city in the twilight, dim and vast, with smoking roofs, soft bells, and gleaming lights." This beautiful picture is a secret clue. The *past* often feels that way. Events that happened a long time ago are not as clear and bright as what is happening right now in the present. They are a little "dim and vast," like a faraway city seen at dusk. You remember the general feeling, the "sounds and sights," but the sharp details have softened over time. You can never go back to that city or that moment. Time only moves forward. The moment you are living right now will quickly be beneath you as you keep going up the hill of your life. Every second that goes by becomes part of your *past*, which you can always remember, but can never visit again. The riddle is smart because it uses a physical place to help us understand a big idea about time. It captures the feeling of looking back on something that is distant yet full of memories.