But trying to influence the world's oil market is a different kind of challenge, experts say.

That was all a kind of challenge, a risk he enjoyed taking.

Perhaps a hundred, and each a different kind of challenge.

It was just a different kind of challenge, one that she hadn't tried before.

And the only kind of challenge worth facing was a tough one.

This was the kind of challenge that gave his life meaning!

"I think they felt this was some kind of challenge."

He needed this kind of challenge to set his blood moving and to test himself.

If this kind of challenge is what you're looking for, move quickly, it will be your last chance.

However, they're still fun, and represent a different kind of challenge.