For his birthday, Leo had gotten Jason a Superman costume. Almost every day, he'd fly around Camp Half Blood, with his cape billowing behind him, yelling, "Don't worry, I'll save you, people of Gotham City!"
This always caused people to laugh, partly because it was nice to see the serious son of Jupiter goofing off, but mainly due to the fact that he couldn't keep his superheroes straight.
On this particular day, Percy had had enough of this disgrace to superheroism. He pulled Jason aside and said,
"Dude, you know Gotham City is in Batman."
The look on Jason's face was priceless.