Drawing a circle and a straight line

Richie loves colors and he can recognize and name most of them. He usually scribbles and makes a circular drawing. Yesterday was different. In the morning, he attempted a full circle without overwriting it .
In the evening, dada and he were playing with colors when I was in the shower. Dada did some straight lines with him with all the different colors and then went to do his work. When I came back, I saw him picking the "purple color" and with the sound of the train "zhuk-zhuk-duku -duku" he draw a straight line. It felt great. He could distinguish the circle from straight line. Another feather to his cap.