We have an expression "teaching someone to suck eggs". The meaning is that you are telling someone how to do something which is obvious or known to everyone. The irony is that most people don't know how to suck eggs anymore. So really the meaning should have come full circle, or do I mean 180 degrees? I am quite interested in the root of old expressions and especially those that are commonly used but we have generally forgotten where they came from and what they originally meant. Any examples?