There is a verse that reads, “When Pharaoh shall speak to you, saying, T’nu lachem mofet, ‘Show a miracle for you’, then you shall say to Aaron, ‘Take your rod and cast it down before Pharaoh, so that it becomes a snake'” (Ex. 7:9).
Rabbi Mendel thought Pharaoh would have said, “Show me a miracle”. Instead he said, “Show a miracle for you”.
Pharaoh was more of a psychologist than we might have imagined. He sensed that some of the Israelites themselves were wavering in their faith. The miracle was not only for Pharaoh but also for the people of Israel.
Both, for quite different reasons, needed a demonstration that God really was powerful and that Moses and Aaron really were His accredited messengers.
In a sense the lesson has suddenly become specially relevant in our own day, when we need to be certain that any wavering spirits in our own midst do not give way to doubt and despair.