(Jon of the Clue Crew shows a line segment sweeping out a circle on the monitor.)  Euclid's third postulate tells us that from any line segment, a circle can be drawn with one endpoint as the center & the segment itself as this line
[DOC] [TLE] Euclid's Postulates -- from Wolfram MathWorld [PAR] Given any straight line segment, a circle can be drawn having the segment as 
radius and one endpoint as center.