So, the Fire Lord, who self-admittedly doesn't know much about spirits, was in some backwater village dealing with a spirit by himself? Slade's not buying it.

"I trust you remember from your experiences with Toph that metalbenders trained by the Beifong Academy can also detect when someone is being less than honest, so... why don't try again. Who else was there?"

The lie-detection claim is, ironically, a lie, but Slade thinks it is one Zuko is likely to believe under the circumstances.