Sentence:  We explain to somebody what is a regular quadrilateral constructed within the circle; then a regular triangle and a regular bi-angle. Now we ask him to draw a regular <WSD>monogon</WSD> by analogy, and we probably think that he cannot do this. But what if he draws a point on the circle and says that it is a regular <WSD>monogon</WSD>? 
Meaning of the word marked with <WSD> tags:  A one-dimensional object comprising one vertex and one (not necessarily straight) edge both of whose ends are that vertex.