Can you answer this riddle?

[ghidorah5464]

This is a riddle none of my friends or teachers have been able to figure out. First one to answer it correctly gets the 10 points.

"Forwards it is heavy. Backwards, it is not. What is it?"

 

[Nell]

Ton. I have heard it before. Sorry for the spoiler

 

[Charger21]

a ton

foward it weighs a ton backwards is not
\when you spell ton backwards it is not