The solution to the riddle “Forward I’m heavy, but backwards I’m not” is one tonne. A tonne spelt backwards is “not”; so, while a tonne is hefty, weighing 2,000 pounds in reality, when looking at the word instead of the thing itself, it becomes “not”
This conundrum belongs to the widespread category of “What am I?” riddles, in which one is given information about an object and must determine what the object is. Answering “I am the beginning of every end and the end of every race” with the letter e is another typical puzzle in this category. These types of riddles frequently involve language play.