Fowards, I'm Heavy




Fowards, I'm Heavy. Backwards I'm Not. What Am I? 
   |
   |
   |
   |
   |
   |
   |
   |
   |
   |
   |
   |
   |
   |
   |
   |
   |
   |
   |
   |
   |
   |
   |
   |
   |
   |
   v
Answer: 

Ton