I am looking for puzzle/bets that you can do with one or two pieces of ropes/strings (like the one where you challenge the audience to tie a knot without letting go of the ends - by crossing you arms)
Any ideas?

Thanks

Kristian
Denmark

Try the classic G. W. Hunter knot. In this routine, the person is doing the same thing that you're doing, but they never wind up with a knot, while you do.

When doing this, there are a couple of things you can do to confuse the issue. For example, just before they toss the rope over their hands, take the ends from them. The rope will form into a knot, and you can congratulate them on having done it correctly! As they're not sure what they did differently this time, they'll start wondering why the same set of actions results in a knot sometimes, but not others.

Thanks Scott

This is exactly the kind of routines I am looking for!
Do you know where the Huntor knot explained?
I am about the do a workshop where I will put some puzzle in with a string/rope...that the spectator can do.
Thanks
Kristian
Denmark

You can find it in Encyclopedia of Rope Magic or Daryl's Expert Rope Magic Made Easy, among other sources. Just about any magic book that extensively covers rope should have it, although they won't always call it the G.W. Hunter Knot.