The Jews were great ones for using various oaths to back up their statements. They were careful, however, not to use the name of God in their oaths, lest they blaspheme God. So, they would swear by heaven, or earth, or Jerusalem, or even by their own heads! But Jesus taught that it is impossible to avoid God in such oaths. Heaven is [God’s] throne, earth is His footstool, and Jerusalem is the “city of the great King.” As for swearing by your head, what good is it? “[You can’t] make one hair black or white”—or even keep one hair on your head.
 Wiersbe, W. W. (1996). The Bible exposition commentary (Vol. 2, p. 381). Wheaton, IL: Victor Books.