Come again??

Today at the Perkins pool, AJ was holding on to the edge and pulling himself around the pool, being a choochoo train. "Woowoo!" he says.
"mama! Come be a train with me!"

I slip in the pool and start pulling myself around the edge with him. "woowoo!" i say.

"no mama. *I'm* the engine. You go back"

I go around him to the other side so he can lead and be the engine.
"that's it, mama. good job!"

"woowoo!" i say. "here comes the train!"
"no mama. I'm the engine. you don't say 'woowoo'. just me"
"oh. sorry. what do i do?"

"you be quiet" he says. "you're the bacoose" he states firmly.
"the what?"
"the bacoose"
"the caboose?" i ask.
"NO. the. bacoose"

right.