It's fast, it's easy - it's magic!
Three ordinary ropes (yes! completely ungimmicked!!)- one long, one medium and one short. In your hands, these three ropes suddenly and visibly all become the same size- you even show them to the audience one by one!
Just as magicially, you return them to their original lengths again. And best of all- you can end the trick by handing the ropes to your spectators- there is nothing to hide