Why is this the answer?

The answer is anchor because this heavy, special tool is exactly what sailors use to make their big ship stop and stay perfectly still on the water. This fun riddle is very clever because it gives us great clues about the two most important actions an anchor is involved in every time a boat travels. Imagine you are sailing a huge boat out on a giant ocean or a big lake. You cannot just use brakes to stop, like you do in a car. If you simply turn off the motor, the wind and the waves will still push the boat around, and you will drift away from where you want to be. To stop safely, you need a very heavy, special tool that will grab the bottom of the water and hold the entire ship in place. That special tool is the anchor. Let us look at the first part of the riddle's puzzle: "What do you throw out to use?" When the boat needs to stop at a certain spot, a sailor takes the anchor, which is usually made of very heavy metal and has big, curved arms, and drops it over the side of the ship. They "throw it out" with a long chain or rope. It falls deep, deep into the water until it reaches the sand or mud at the bottom. The heavy metal digs in, and that holds the enormous ship still, like a giant, underwater hand. Now for the second part of the riddle: "and take in when you're done?" When the crew is finished resting or exploring in that spot, and they are ready to sail away for a new adventure, they must pull the anchor back up onto the boat. They use a strong motor or crank to lift the heavy chain and the anchor all the way out of the water. When it is safely back resting on the deck of the ship, they have "taken it in" or brought it back on board. The boat is now free to move! So, they throw it out to stop, and they take it in to go. That makes the anchor the perfect answer to this sea puzzle.