My geometry book asks me to distinguish the two terms. First, though, I noted that you cannot have a half of a line becuase lines stretch indefinitely, isn't that correct? If so, did they possible mean segment? Because in that case, the half of a line segment would be a midpoint, and therein would the lie the distinction, that the midpoint is not a vertex. Am i mistaken, or missing anything? Or are my thoughts utterly wrong?