The Tale of the "Big" and the "Small" Ship

During his first trip aboard a "big" ship, a ferry to be exact, to carry us to a neighboring island, my 41/2-year-old was looking out the window the whole time and was pressing his ears against the glass to hear the sound of the sailing ship. While the ferry was inching its way along the crowded wharf, he suddenly blurted,
"Look! Mama, a small boat pulling a big ship!"
He said, pointing to a cargo ship pulled by a tugboat.
He turned to me with a puzzled look, so, I hurriedly explained,
"It's a tugboat, Chase. It's small but strong. It's pulling the big boat because the big boat can't turn on its engine yet. "
"Why?" he asked. I was bracing myself with his litany of "whys."
It's frustrating. How can you make a 4-year-old understand things like this?
"Chase, the wharf is crowded or  maybe the water is too shallow or big ships are not supposed to turn on their engine while at the wharf."
He just looked at me and returned to look out the window. I was hoping he understood my somewhat cluttered explanation when he again turned to me. I was expecting another "why" when he slowly said,
"No, Mama, it's a young ship helping the old ship."