A sequence of characters, either as a literal constant or as some kind of variable, that can have its elements be mutated and length changed.
Why do I see duplicate challenges? You'll see duplicates because a question may fall into different categories. We keep them here bec ause we believe that it is good to sporadically review old challenges. They are optional if already completd-- but reviewing them (and more importantly redoing them with the new knowledge) will ensure that you've let the material sink in.