Exodus 3:20 (ESV)

20 So I will stretch out my hand and strike Egypt with all the wonders that I will do in it; after that he will let you go.

God can give a sign that is no wonder, for example, Exodus 3:12 where the sign is fulfilled by natural means, that is, a walking people (Israel walking out of Egypt as a sign). A wonder is a supernatural event or manifestation of divine power (e.g., Exodus 4:8. God gives a preview to Moses of the wonders (plagues) of Egypt (Exodus 7:1 – 11:10).