iconography - What's a good icon to represent how difficult a task is to do?

I am working on an application where users get to rate various things. In one category, they are rating how "good" something is, and so I put together a little 1-5 rating system with star icons, like Netflix has.

But now I need a way for users to rate how difficult a task is, again from 1-5. I'd like to use the same sort of UI element, ie users can select from 1-5 of something to enter their selection. What is a good icon to use for this?

P.S. I know this question is related, but as this is a business application, I don't think I should use icons of skulls or daggers. =)

Answer

I think that color will work better here than icon shape.