How lines work

Which one is a straight line?  Wait.  Don't just answer off the top of your head.  The answer to this question is apparently trickier than it seems.  Ok, did you pick the middle one?  If you did, congratulations, you can identify a line and appreciate this post.

Do you ever have to stand line?  I do it at the Daley Center every day.  It's one of the perks of my job.  Every day I'm amazed by the inability of people to understand how lines work.  Should you stand next to me?  No.  Should you jump in front of me? No.  Should you curve off in a different direction?  No!  You belong behind me, conforming to the original direction of the line.

This is a kindergarten concept, guys.  Kindergarten.