Why is 180 sometimes called straight angle?

Answer 1

Because if there's no little dot to mark the vertex of a 180-degree angle,

then you don't even know that you're looking at an angle, because it looks

just like a straight line.

To put it the other way, if somebody tells you to construct a 180-degree angle

with nothing but a ruler, all you have to do is draw a straight line and mark a

little dot anywhere on it, and boom, you've got a 180-degree angle with its

vertex located at that dot.