Why is this the answer?

The answer is sand because this clever riddle uses two very different but important ways that we think about time to trick you. First, let us look at the beginning of the riddle: "What measures out time." To understand this, we need to think about a special tool called an hourglass, or a sand timer. You might have one in your kitchen to time how long you brush your teeth or how long a game lasts. An hourglass is made of glass and has two round parts connected in the middle by a tiny opening. Inside, it holds a perfect amount of tiny grains of sand. When you turn the hourglass over, the sand slowly falls from the top section to the bottom section. The exact amount of time it takes for all the sand to finish falling is how the hourglass measures time. It is a very old and reliable way that people have used for hundreds of years to keep track of moments. That is why the riddle says sand "measures out time." Now for the second, very clever part: "Until in time all is smashed to it." This part of the riddle is talking about nature and a huge, huge amount of time, like millions of years. Think about the biggest, toughest rock you can find, maybe on a beach or a tall mountain. Even though rocks seem super strong, they cannot stay that way forever. When the strong winds blow on them, or the ocean waves crash against them, or the rain falls on them year after year, the rock slowly begins to break down into smaller and smaller pieces. This wearing away process is called erosion. Everything in nature, including big boulders, old statues, and mountains, will eventually be slowly ground up into tiny specks. These tiny specks are the final, fine grains of sand. The riddle says everything is "smashed to it" because, given enough time, nature turns almost all solid materials into sand. So, sand is both a tool that shows us small amounts of time passing and the tiny piece that everything becomes after a very long time.