The ancient painting

In an Aboriginal Studies class, the teacher was saying how amazing the aboriginal culture contributions are. As an example he told us about ancient paintings. He said he did a trip to a lake to see the paintings on the side of a cliff and that it was hard to imagine how the aboriginal artist could draw such perfect lines, the boat would be moving a lot. He continued his class but came back to the quality of the line, saying that maybe the artist had climb down from the top like a window washer. He asked the opinion of the students. Nobody seemed to disagree. When he asked an aboriginal student, the quiet student answered: "I don't know but he painted it on winter, when he could just walk on the lake".