Recognizing the symbol, another person to identify himself/herself as a Christian would complete the second (or bottom) half of the fish.
But why the symbol of a fish? We know that among the earliest Christian symbols was an anchor or a peacock (symbolizing new life). Why a fish? Simply because the word for fish in Greek is ichthys, spelled with 5 letters in Greek (the english equivalent would look like this: i-ch-th-y-s and these 5 letters formed an accrostic meaning to these early Christian believers (that is, each of the five letters stood for a word that embodied their faith). Here was their declaration: Iesous Christos Theou Yios Soter, which means "Jesus Christ, God, Son, Savior!" So next time, smile at that fish you see in the aquarium, and say, "I like fish--ALOT!"