The WHY phase.
Sorry, that is the brat inside coming out....
My DD hasnt hit this phase yet, but my bff's kid is in it now, and it's been a little over a year. She's starting to accept the answers, "Why do you think?" and "Ill answer "why" three times and that's all". I can imagine that it would be really annoying.
Ds was in it strong from age 3 until just about his 7th birthday. Ds also didn't respond well to "why do you think?" He really wanted to know. Part of it, I'm sure, was that he didn't have other great conversational openings. Once he figured those out, they did lessen. But part of it is, he's always wanted to understand. Today, at 10, he really wants to understand current events. (Try answering "why" for what's going on in Syria!) It's part of his personality.
Be thankful that he's got an inquisitive mind. Master the phrase "I need 5 minutes of quiet so my mind can rest. No questions for 5 minutes please." (and yes, be prepared for him to ask "why?" for that one too!)
This kicked in for DD when she turned 3 (last October) and it hasn't let up yet. Sometimes she doesn't listen to our answer and just asks "why" again. So we have to set some limits to how many times we'll answer the same question.
Hang in there, mama. This phase can't last forever, right?