I have an unfortunate predisposition towards sarcasm, and a friend suggested I had a flag to indicate when I was being sincere. This virtual flag is used to signal to my friends and colleagues when I truly mean something. For example 'nice jumper' may mean, 'that's nice, where could I buy one for myself?' Or - 'Yuck! How can you go out wearing that thing?' So to avoid confusion I sometimes have to indicate 'sincerity flag up' or 'sincerity flag down'. Until recently I thought I was the only one who had such a thing, but then a friend of my sister recently waved her 'only joking' flag which is pretty much the same concept.