“millstone turned by a donkey.” The Greek literally reads, “millstone of a donkey,” and it refers to the large commercial millstones, which weighed many hundreds of pounds and were turned by donkeys or oxen; see commentary on Mark 9:42.
“the lake.” At that time Jesus and the disciples were in Galilee, and near the Sea of Galilee, which is a lake. The name, “the Sea of Galilee,” was given by people who had never seen it. It is actually quite a small lake. Using “the sea” here makes Jesus’ statement much more general and removes it from its geographical context.