Short story from the Percy Jackson series that's free to read on the author's website. Percy escapes science class to help retrieve a stolen chariot from a zoo, and it's as chaos-inducing as it sounds (the otters are not happy, although an escaped leopard does get to snack on some pigeons so at least something is). Granted, I've only read a couple of books in the series but I don't think there's anything in here that's necessary to read to understand what's going on in the novels; it's a fun bonus more than anything else. I enjoyed it.