Why is this the answer?

The answer is time because this clever riddle describes something we use every single day that is both invisible and incredibly important. Time is like a secret code for how long things last, but it is hard to notice it until you start paying attention. Let's look at the first clue: "Until I am measured I am not known." When we measure things, like your height or the amount of milk in a glass, we use special tools like rulers or measuring cups. For time, we use things like clocks, watches, and calendars. Before people invented these tools to count the seconds, minutes, and hours, time was just something that happened. We only knew it by the sun coming up and going down, or by the changing seasons. But now that we measure it, we know exactly how long our school day is, or how many weeks until a holiday. The riddle is saying that to truly understand how much time we have, we have to use our clever inventions to measure it and make it "known." Now let's look at the second clue: "Yet how you miss me when I have flown." This part is all about how you feel inside. Think about when you are having a wonderful time playing your favorite game, spending time with a friend, or reading an exciting book. In those moments, it feels like the minutes zip by so quickly! The word "flown" means that the time has passed very fast, just like a bird soaring high in the sky. When the fun is over, you might sigh and wish you had more time. This is when you "miss" it. Maybe you wish you could have those happy moments back because they passed too soon. Time is the one thing that keeps moving forward and never stops, which is what makes it so precious and why we notice it most after it is already gone. That is the tricky, wonderful secret of time!