My brother (who is 2 years younger) and I have our own language, consisting of movie quotes and silly stuff we made up when we were little. We speak it with abandon when we're alone, and try as hard as we can to suppress it when we're with a friend. But sometimes some of it slips out, and the friend looks at us like he's the guy who isn't in on the joke; I always get the feeling he or she thinks the two of us are crazy. By the way, if you're the third person in company with two close siblings who are speaking their own secret language, don't ask them to explain or look at them like they're crazy. They're not nuts, and you won't understand, even if they try to explain. Just let it pass.