Because

Gavin (concentrating as he plays with his little legos): "Mom, I can't make this airplane!"

Me (reading a book while sitting on the couch): "You have to keep trying.  You'll only get better through practice."

Gavin (pleading): "MOMMYYYYY! Come on!  Just help me!"

Me (without looking up): "Gavin, Mommy isn't the lego expert builder."

Gavin (holding some legos in front of my face): "Come on.  You have to."

Me (still reading): "I have to?  And why is that?"

Gavin: "Because you're my Mommy.  And you're 'posed to help me."

5 seconds of silence

Me (putting the book down): "Give me the legos."