"Where's Lily's Mom?"

This was Evan's question to me today - "Where's Lily's mom?" So was Evan thinking that Lily was here temporarily? We were just watching her while her mom was on vacation? This question confused me because Evan always explains how Lily came out of my belly and now my belly is "empty". Thus I thought he understood but I guess not! I explained to him that I am Lily's mom just like I am his mom. I am a mom to both of them just like dad is both of their dads. Well after this explanation I think Evan has it figured out because once Rob came home from school Evan explained it all to him.