Some sensible phrases are practically impossible. The phrase “watch your head” is a perfect example. I cannot observe my head without a mirror; and looking at the reflection ignores the impending danger. Yet we say it, anyway.
Consider the phrase “watch out.” There is no other direction one can watch. Since eyes are always pointed away from oneself, the phrase seems redundant. Yet the phrase communicates an object requiring special attention.
Jesus taught His followers to follow Him. First, He emphasized the need for authenticity. Next, He underscored that treasure resides where the heart abides. Now, He demands vigilance. Faithful attention is required. Faithfulness is displayed in waiting and working.