I guarantee he brings up the whole discovering-fire thing within the first five minutes.
A joke about how couples sometimes gossip. While the wives are friends, the men in these two couples are apparently in a bit of a feud -- one built an impressive tower, whereas, the other invented fire. The tower builder is jealous of the fire inventor, and worries he'll bring up his invention immediately. It's funny in part because spouses sometimes have to put up with their partner's friends, even though they don't like them.