Forward I am heavy, but backward I am not. What am I?

How to think about it

1. “Forward I am heavy” suggests a word that means a large weight.
2. Try the short ones: pound, ton, load.
3. Reverse each. “Pound” backward is nonsense, but “ton” backward is “not”.
4. “Backward I am not” is literally true: t-o-n reversed spells n-o-t.

Why “Ton” is the answer

A ton is 2,000 pounds, which is heavy, and the word spelled backward is “not”, so the riddle’s second line is a literal description of the reversal. The riddle hides a palindrome-style trick inside a sentence that sounds like a boast. Once you reverse “ton”, the pun is unmistakable, and the hint to spell it backward gives the method away.

Common wrong answers

• A truck. A truck is heavy forward and backward; the riddle is about letters, not vehicles.
• Pound. Pound is heavy, but “dnuop” means nothing, so the backward half of the riddle fails.