Joel is going to get the names of rich people who will be going to a feast; while they are out, Rosh will rob their houses, furthering his cause. However, despite a good plan to sell fish to the slaves of the rich, Joel cannot figure out how to do the job without being suspected. Thacia comes up with a plan; she will wear his clothing, and pretend to be him, therefore freeing him to act without suspision.
"Thace wears my clothes, and with her hair back under the turban who's to know she's not a boy? Then you two go about together. I'll arrange for a day off. You could go -- oh -- somewhere out of the city would be best -- where we're not too well known."
(Speare, The Bronze Bow, Google Books)
This deception sets up an event in which Daniel learns an important lesson about picking his battles; it also allows Thacia and Daniel to spend time alone together, deepening the innate feelings they have for each other. The swapped-clothing deception is a classic literary trope, appearing as early as Mark Twain's 1881 novel The Prince and the Pauper; here, it is used to serious effect, rather than comedic, as both Daniel and Thacia could be executed for their deception.