Ainsley started the "why" phase 6 months ago, but it is big & bad at present.
Here are two of our favorite "why" incidents thus far:

1) While I'm changing Max's diaper -
"Mommy! Max is grabbing his
penis!!" Ainsley proclaims.
"Yes, I know he is, Ainsley."
"But why is he doing that?"
"I'm not sure," because really, I'm not. Are you? Because if you do know & know how I can discourage this, please let me know.
"But WHY?"
"I don't know, honey."
"But WHHHHHY?"
"I don't know, sometimes he just does." (Brad is stifling laughter from the couch as I shoot him a "help me & please-explain-to-your-daughter-why-your-inferior-gender-needs-to-grab-your-genitalia-all-the-time look.")
"BUUUT WHHHHHHHHHHY? WHHHHHHHHY IS MAX TOUCHING HIS PENIS, MOMMMMMMMMMY?"
"Ainsley, let's go outside," Brad says.
Daddy saves the day. For now.
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2) While Ainsley is going through my purse:
"What's this Mommy?" as she pulls a tampon out of my purse.
"That's one of Mommy's private things."
"But what IS it?"
"One of my private things."
"BUT WHAT IS IT???!!!!"
"Ainsley, it is one of my private things. When you are a mommy you will have it too, now please put it back in my purse."
"UGH!!!!"
I am sure tears followed this, but since that happens about 10 times a day, I really didn't pay attention.
WHY????