"Mommy, what's wrong? You OK?"
"Yes, I'm OK, just a little sad."
"Mommy, why are you sad?"
"I'm just missing my friend."
"Don't be sad, we have trains! Here you can play with my Rosie train." (This of course makes me tear up a little, which makes Lucas give me a big hug) "Mommy, it's okay. Don't cry. I'm here. You're my best friend and I'm your best friend."