Why is this the answer?

The answer is iron because this clever riddle describes the secret work of a blacksmith! A blacksmith is a special kind of worker who makes tools and other useful things out of tough, strong metal, like iron. This riddle is a fantastic word picture that shows us the amazing changes that iron goes through when the blacksmith heats it up and then cools it down very quickly. Let's look closely at the first part of the riddle: "What goes in the water red". This clue is talking about the iron after the blacksmith has put it into a very hot furnace or forge. The forge is a special oven that uses fire to make the iron extremely hot. When the iron gets super, super hot, it begins to glow a bright, fiery red color. This color shows the blacksmith that the metal is soft enough to work with. They can now take their big hammer and shape the iron into whatever they need, like a horseshoe, a new latch for a gate, or a useful garden tool. The blacksmith must work very quickly while the metal is this glowing red color, because the metal will start to cool off and get hard again right away. When the blacksmith is completely done shaping the hot, red metal, they need to make it strong and stiff so the new tool will last a long time. This is where the second part of the riddle comes in: "and comes out black". To make the hot iron strong and cool, the blacksmith dips it straight into a big tank or tub of water. This quick cooling process is called "quenching". When the glowing red iron hits the water, there is always a big, exciting hiss and a giant puff of steam. The water pulls the heat out of the iron incredibly fast. When the iron comes out of the water, it is no longer glowing red. Instead, all that intense heat leaves behind a thin layer of dark, powdery scale on the surface, which makes the iron look completely black. So, the iron went in red from the fire, and it came out black, cooled, and very strong. The riddle is a cool, sneaky way to talk about the magical job of a blacksmith!