"The Jesus Fish" or the "Ichthys" in Greek, is often seen these days in the form of car decals and bracelet charms. The symbol became associated with Christians when, during the first century, the early Christians would draw one half of the symbol, and if a stranger completed the fish, he was a Christian and a friend, and if he didn't, he was not to be trusted.
But, I never stopped to wonder why they used the fish. I always assumed that it had something to do with Christ's multiplication of loaves and fishes, but, while talking with one of my former teachers, I learned that there was a greater reason for the fish becoming "the" Christian ID symbol.
Iota (i) is the first letter of Iēsous (Ἰησοῦς), Greek for "Jesus".
Chi (ch) is the first letter of Christos (Χριστός), Greek for "anointed".
Theta (th) is the first letter of Theou (Θεοῦ), Greek for "God's", the genitive case of Θεóς, Theos, Greek for "God".
Ypsilon (y) is the first letter of (h)yios (Υἱός), Greek for "Son".
Sigma (s) is the first letter of sōtēr (Σωτήρ), Greek for "Savior"."
So, there's your history, language, and religion lesson for today! Fascinating, isn't it?
May "Ichthys" bless all of you today! :)
The image and quoted text about the fish are both from Wikipedia.