Trains and Jackhammers

Yesterday morning during coffee, Joseph was quietly eating a Graham cracker. All of a sudden I heard him say 'chukka- chukka- chukka, whoo, whoo,' and while doing this was making his cracker move along the front edge of his highchair. And if this wasn't entertaining enough (for me, not him) during the next cracker he started tapping his cracker into the tray, saying 'dat, dat, dat, dat, jack hammer.' (When he says jackhammer, he makes it like two words.)
Now, you may be wondering 'how did he come to know what a jackhammer is?' After all, I wouldn't think that the word 'jackhammer' is something that most one and a half year old toddlers would know. Well, at the moment Joseph's favorite book is 'The Construction Alphabet Book' by Jerry Palotta, and J is for Jackhammer. When we were reading the book one day he became fixated on the jackhammer, so I held him in front of me by his thighs, making him jump up and down and saying 'dat dat dat dat.' Then I said 'that is like a Jackhammer'. He loved it and now sometimes he grabs hold of his pant legs and declares 'Jackhammer! dat dat dat dat.' I was really surprised, however, to see that him transfer that experience to understating that the Jackhammer is a object that makes a sound.