The Egyptians will know that I am the Lord when I stretch out my hand against Egypt

 Exodus 7-8

 You are to say everything I command you, and your brother Aaron is to tell Pharaoh to let the Israelites go out of his country. But I will harden Pharaoh’s heart, and though I multiply my signs and wonders in Egypt, he will not listen to you. Then I will lay my hand on Egypt and with mighty acts of judgment I will bring out my divisions, my people the Israelites. And the Egyptians will know that I am the Lord when I stretch out my hand against Egypt and bring the Israelites out of it.”

Moses was eighty years old and Aaron eighty-three when they spoke to Pharaoh.

Each one threw down his staff and it became a snake. But Aaron’s staff swallowed up their staffs. Yet Pharaoh’s heart became hard and he would not listen to them, just as the Lord had said.

With the staff that is in my hand I will strike the water of the Nile, and it will be changed into blood. The fish in the Nile will die, and the river will stink; the Egyptians will not be able to drink its water.

Then the Lord said to Moses, “Tell Aaron, ‘Stretch out your hand with your staff over the streams and canals and ponds, and make frogs come up on the land of Egypt.

All the dust throughout the land of Egypt became gnats. 

The Plague of Flies.

The houses of the Egyptians will be full of flies; even the ground will be covered with them.

Why?

The Lord gave Egypt nine nature disasters, but they were still harden their heart, until the tenth diaster happened.

How?

The word of the Lord are always come around us, do I sense it and make my soul awake to listen the word of the Lord?