Why is this the answer?

The answer is breath because this clever riddle uses simple clues to describe something that is super important for every single person on Earth. We can figure out the answer by looking at each part of the riddle. The first part says, "I am free for the taking." Think about what you use all day long without having to buy it or ask permission. That is the air we breathe! It is everywhere around us, ready for our lungs whenever we need it, so it is definitely free. We never have to stop and think about where to find our next breath; it is always right there. Next, the riddle gives a tricky clue: "Through all of your life, though given but once at birth." We start to breathe the moment we are born, taking our very first gasp of air. That first breath is the one that begins our entire life of breathing, which we then do all the time, without stopping, until the end. This is what the riddle means by being "given but once at birth." The third clue mentions that it is "less than nothing in weight." This is a fun way to describe air. If you hold a breath in your hand, you cannot feel any weight, right? That is because air is so light it feels like it weighs nothing at all, even though the world is completely full of it. Finally, the last part of the riddle is the best clue: "but will fell the strongest of you if held." To "fell" someone means to knock them down. If you try to hold your breath for a long time, even a super-strong grownup will get dizzy and pass out. This happens because your body needs that free air to keep working. The strongest person in the world cannot hold their breath forever. This whole riddle is a very smart puzzle that tricks your brain into thinking about a small word with a huge job!