根据上述内容，判断下列句子正误。

Mark wanted to learn painting. His grandfather told him to paint a foot at first. He wanted Mark to watch the model carefully and then paint it on the paper. When his grandfather left the room, Mark looked at the model for a while. Then he took off his shoes and looked at his own feet. Suddenly he found an easy way to paint, Mark made his feet black with the paint and walked on the paper. At once, his footprint(脚印)were left on the paper. Soon his grandfather came back and was angry with him, but Mark proudly said that painting the feet was the easiest thing in the world.