Why is this the answer?

The answer is shoe because this clever riddle uses a fun word trick to describe a very common object that you wear every single day. The person who wrote this riddle was playing a special game with words to make you think hard about two different things that share the same name. Let us look closely at the first part of the riddle: "Iron on a mammal." To solve this, you need to think about a big, strong animal that has been very helpful to people for thousands of years. That animal is a horse. A horse is a type of mammal, just like a dog, a cat, or even you and your classmates! Now, what do people sometimes put on the feet of a horse to protect them when they run or walk on hard roads or rocky paths? It is a piece of curved iron called a horseshoe. Horseshoes are special because they keep the horse's hooves safe and sound. So, the "iron" is the horseshoe itself, and the "mammal" is the horse wearing it. The big secret to the riddle is that the word "shoe" is hiding right inside the word "horseshoe." It is a word within a word, which is a classic riddle trick! Now we move to the second, more direct clue: "Mammal on your feet." This clue is a bit silly and tries to trick you! It makes you picture an actual animal sitting on your foot, which is not what you are supposed to think about. Instead, the riddle is asking you to think about the common object that you wear every single day that is made to protect and cover your own feet. That object, of course, is a shoe. A shoe is the item that is always on your feet when you are outside playing or inside walking. When you put both clues together, the riddle points to the word that is both a part of the horse's protective iron accessory and the name of what you put on your own foot. That makes "shoe" the perfect answer that links the first part of the riddle's smart wordplay with the second part's simple, everyday object. It is a wonderfully tricky puzzle!