B-F-G

This is a classic, and one of the first... I cannot remember the specific context of this story, I think it went a little like this... We were camping with family and were talking about my step-brother needing new glasses because he keeps breaking them.
 Rick to tells my parents that they should get Christopher some BFG's, cause at least they aren't as easy to break as those flimsy ones he wears. We all furrow our brows thinking "BFG!? What is that?" So we ask him him what he means, to which my husband replies: "Birf Control Glasses." Long pause  as we think about what he just saids, someone asks "what's the F stand for?" I quickly run through these sayings in my head Birth Control Glasses, Big F*&^ing Glasses, trying to figure out what's wrong with this statement. My husband says "Birf Control Glasses", again we ask "yeah but whats the F stand for?" SO he repeats himself, this time drawing out each syllable "Read my lips BIRFFFFFFF Control Glasses," and we all burst into laughter! To which he looks at us confused... It sure did take a minute to stop laughing enough to explain where he went wrong, after several attempts ending in laughing some more, at this point he wanted to know what was up; I finally told him what he meant to say.

The real saying: Birth Control Glasses aka BCG.