According to pragmatics linguists, you tend to be around five or six years old when you start to understand that saying things opposite to the self-evident truth can be funny. Hearing people or people who use their voices automatically associate this with drawls, deadpan monotones or emphasized intonation. The same thing happens in sign language. Irony isn’t just auditory, after all—it’s also in facial expressions and body language. The face easily shows excessive excitement or flat affect, and equally common is the tongue-in-cheek flourish of some signs or a mock-serious leaning in and furrowing of the brows as someone says “Really?!” Given that facial expressions are a natural part of the grammar of any signed language, it requires a certain understanding of what the person...