Why is this the answer?

The answer is ton because this is a super sneaky word trick that plays with how you read and spell! When you hear the word "ton" in real life, you usually think of something very, very heavy. A ton is a huge measurement of weight that people use for very large things, like a giant truck, a full-grown hippopotamus, or a whole pile of massive stones. The word T-O-N is all about extreme heaviness. So, the first part of the riddle, "Forward I'm heavy," is telling you to think about this giant weight. When you spell the word "ton" out loud, that is reading it forward, and its true meaning is definitely about something heavy. That first clue is a great hint about the word's meaning. But then the riddle has a clever second part that tries to fool you: "but backwards I'm not." To figure out this clue, you have to stop thinking about the word's meaning and start thinking about its letters. You need to take the letters of T-O-N and spell them backward, starting with the last letter. If you start at the end and go to the beginning, T-O-N spells N-O-T. The word "not" means the opposite of something, and in this riddle, it means the opposite of heavy. If something is "not" heavy, that means it is completely light, or it doesn't weigh anything at all! This riddle is a classic puzzle because it makes you look at a very common word in two different ways. First, you look at what the word T-O-N means, and that makes you think of great weight. Then, you look at the letters and spell them backwards to get a brand new word, N-O-T, which gives you the final answer to the trick. It is a fantastic example of a riddle that uses simple words in a really smart way to create a fun brain teaser. You have to be a great listener and a great speller to solve this one by reading the word both ways.