What episode of avatar do Katara and Sokka give fake names?

Katara and Sokka give fake names in Episode 17 of Avatar: The Last Airbender, "The Great Divide".

They use the names "Kya" and "Sokka" (yes, Sokka uses his real name!) to avoid suspicion from the Earth Kingdom citizens who are suspicious of anyone who looks like they could be from the Fire Nation.