cozie I take the 5th on whether or not I've already cracked the
this morning. 
Taking the 5th is done to avoid making statements that 'may' incriminate yourself. It pretty much says 'yeah, I did something illegal but I'm not going to tell you about it'. It doesn't just mean that you've done what they're asking about, it could mean that you've done something else illegal that answering the question could lead to, i.e. 'I'm not going to answer your question about XX breaking into the jewelry store because it may lead to the fact that I received the stolen goods'.
Here's what they generally say: “On the advice of counsel I invoke my Fifth Amendment privilege against self-incrimination, and respectfully decline to answer your question.”. To each and every question, separately.