Why is this the answer?

The answer is sand because this clever riddle describes exactly what happens inside a special clock called an hourglass! An hourglass is a fun and simple tool that people use to measure time. It has two glass parts, shaped like bulbs, that are connected by a very narrow opening, and inside, it holds lots and lots of tiny grains of sand. Let's look closely at the first part of the riddle: "One by one we fall from heaven down into the depths of past." Think about the top part of the hourglass as the "heaven," or where the sand starts its journey. Each little grain of sand is a piece of the "we" that is mentioned. When you turn the hourglass over, all those grains start to drop slowly through the tiny hole. They fall "one by one," just as the riddle says. As the sand piles up in the bottom glass, it is marking the time that has passed, which is what the riddle means by "the depths of past." It is a neat way of thinking about how time keeps moving and one moment becomes part of history. The second line helps us confirm the answer: "And our world is ever upturned so that yet some time we’ll last." Once every grain of sand has fallen to the bottom, the time being measured is all gone. The only way to start measuring time again is to take the hourglass and flip it over, or "upturn" it. You move the whole "world" of sand back to the top so it can begin its falling journey once more. By turning it over, you allow the sand to "last" and continue measuring time, sometimes for an hour, half an hour, or any length of time the hourglass was made for. The sand is always falling and the hourglass is always being turned, making it a never-ending cycle of time measurement. This wonderful puzzle uses strong pictures and makes you think about an object people have used for hundreds of years.