Why is this the answer?

The answer is grudge because this clever riddle describes a strong feeling that we carry inside our hearts, not a real, physical object. A grudge is when you stay angry at someone for something they did. Let's look at the clues one by one to see how they fit the word "grudge." The first clue says, "They can be harbored, but few hold water." To "harbor" something means to keep it safe or secretly hold it, like a safe spot for ships. You can hold onto a bad feeling, or harbor a grudge, inside your mind. The part about "few hold water" is a trick! If something "holds water," it means it makes sense or is believable. A grudge, which is staying mad for a long time, often does not make sense or "hold water." The next line is, "You can nurse them, but only by holding them against someone else." When you "nurse" a feeling, it means you keep it going and make it stronger, like how a nurse cares for someone. You keep a grudge alive by thinking about the person and what they did wrong, always holding that anger *against* them. It is all about how you feel toward that other person. The riddle continues with, "You can carry them, but not with your arms." A grudge is a heavy feeling. When you are angry and refuse to forgive, it feels like you are carrying a burden in your heart. You carry this feeling with your thoughts, not with your actual arms or hands. It weighs on your mood and your mind, even though it weighs nothing in the real world. Finally, the riddle says, "You can bury them, but not in the earth." When you finally decide to forgive someone and move on from that bad feeling, you "bury the hatchet," which means you bury the grudge. You put that anger in the past, hidden deep in your memory, instead of digging a hole in the ground for it. All these clues perfectly describe the unhappy feeling we call a grudge.