since when did mothers have to explain their actions to their 3-yr olds?

Yesterday morning, Big Bro asked if I could carry him down the stairs. I told him, "I can't, M. I told you Mommy's knees hurt."

He thought for a moment. "You carry only my sisters?"

Feeling like I've been caught in a lie, "They're babies and they are not very good at doing down the stairs yet. But you're a big boy and can do that by yourself."

He cocked his head to the side and crinkled the side of his face. He looked up at me and asked, "Your knees don't hurt when you carry my sisters?"

I conceded quietly and thought it better to change the subject at this point.