“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. There would be no point in tying a commercial millstone to anyone’s neck and throwing him in the water—any much smaller weight would do the job. Jesus is using hyperbole to make his point. Most translations do not point out that this verse is speaking about the huge commercial millstones and not the standard household millstone that women used to grind the household grain. [For more on millstones, see commentary on Deut. 24:6].
“lake.” From the context, he was teaching in Capernaum, right beside the Sea of Galilee, which is actually a lake.