Why is this the answer?

The answer is doughnut because this riddle is a clever and funny word puzzle that tries to trick your ears and your brain. Riddles often use words that sound like other words, and that is exactly what is happening here. The first part asks about a "nut." When you say the word "doughnut" out loud, it sounds very much like "do-nut." It is a fun play on words. Your mind immediately starts picturing crunchy acorns, peanuts, or walnuts, which are real nuts. This is the riddle's first trick to send you looking in the wrong direction. Then the riddle adds the clue, "empty at the center." This is the best clue of all! Think about what most doughnuts look like when people bring a box home from the store. Many kinds are shaped like a ring or a circle, with a perfectly round hole right in the middle. That hole is the "empty center" the riddle is talking about. It is the perfect description of a very popular sweet treat that you know well. The last part of the riddle is that this "nut" "has no shell." Real nuts have hard, protective shells that you have to crack or peel away before you can eat them. But a doughnut does not have a shell at all. It is a soft, fried dough that might be covered in sugar, cinnamon, or a colorful, sweet glaze. There is no hard outside to break open to get to the delicious part. The riddle is helping you realize that even though it sounds like a real nut, it cannot be one because it is soft and ready to eat. To figure out this riddle, you have to stop thinking about a nut that grows on a tree and start thinking about the word itself. It is a very sweet kind of word trick. You take the "nut" sound, match it with the hole in the middle, and realize the answer is a tasty, shell-free doughnut. It is a great example of how riddles use everyday objects and funny descriptions to make you think hard and solve the puzzle.