ΙΧΘΥΣ is Greek for fish. It is an acronym for
(Iēsous Christos, Theou Huios, Sōtēr)
The letters can be combined to form a spoked wheel, as in this image at Ephesus:
In the days that Christianity was illegal, and you wanted to check if someone might be a Christian, you could take your staff and draw a semicircle in the dust. If the other person drew another semicircle, completing the simple fish – you had probably safely met another Christian.
There are also parodies of the symbol. A lot are working through the (IMO tiresome) Creationist-Evolution “debate”. With variations on which appears to be triumphing. Or the possibility of combining being a Christian and accepting evolution.
Thanks to my friend who suggested that, from time to time, I introduce a sign or symbol or something similar, on this site.