The moth went in because "the light was on", which is vernacular for "open for business", but it has a double meaning because moths are attracted to light at ni

The moth went in because "the light was on", which is vernacular for "open for business", but it has a double meaning because moths are attracted to light at night. The punchline itself is admittedly weak -- the humor comes from the fact that you listened to a lengthy story that turned out to be a really bad joke.