If thou wilt not let my people go, behold, I will send swarms of flies upon thee, and upon thy servants, and upon thy people, and into thy houses: and the houses of the Egyptians shall be full of swarms of flies. Exodus 8:21

Trivia: In Exodus 19:9, God said all the people would hear his voice when he talked to Moses. In that meeting, how did God say he would manifest himself?