Serious conversation with a 6 year old

When I was four my biological father took off. The 'out to buy milk & never came back' story was our story. This happened in 1972, prior to any child support laws or easy way of tracing him. My mom had to work many jobs to support three kids until she met my step father in 1982 - married in 1983. Anyway, last night Brent starts chatting about last names & asked how come my name before I married wasn't the same as Mom-Mom & Poppy's. "Oh boy" I thought, how do I explain this. Here's the rest of the conversation:

Me: "You know how Allison had a mommy before me?"
Brent: "Yeah, she had a China mommy."
Me: "Yes, she had a China mommy. Well, I had a daddy before Poppy. He was sick and couldn't be a good dad, so we never saw him again."
Brent: "Is he dead?"
Me: "No, he's not dead?"
Brent: "So, he's not in heaven with Daddy's mom & dad?"
Me: "No, he's not in heaven. I don't know where he lives?"
Brent: "Why was he sick?"
Me: "Well, he was sick in his head?"
Brent: "Like a headache? Did he need Tylenol?"
Me: "He didn't need Tylenol. He was a little crazy. He took drugs, remember how your school talked to you about not taking drugs? My first dad took the drugs and it made his brain sick - he didn't act normal & people call that crazy."
Brent: "OK."
Me: "If you have any questions about him, please don't be afraid to ask. And, I don't want you to worry about me or Daddy taking drugs or leaving and not coming back. That's not normal and we wouldn't do that. I don't want you to be confused or to worry."
Brent: "I'm not confused. I understand. Your first dad ate bugs and acted like a chicken."
Me: "Huh? Ate bugs? Acted like a chicken?"
Brent: "Yeah, your dad ate bugs and then he acted like a chicken - and that's crazy."
Me: "Yeah, something like that."
Brent: "Wanna see me act like a chicken?" Up from the dinner table he jumped and clucked his heart out. (How sweet is the innocence of a six year old.)

(we'll talk more about saying no to drugs, bugs and slugs later...)