In Season 6, Episode 18’s “The Truth in the Myth,” Brennan cracks a joke about, well, Seeley’s (David Boreanaz) crack. While discussing the veracity of some myths, Booth remarks to his ever-skeptical partner, “You haven’t seen the backside of the moon, but you believe in it.”
Brennan — who can barely contain her giggles — takes a few stabs at a joke about the backside of Booth’s moon. Get it? Though it definitely doesn’t have us doubled over in laughter, Booth reassures her that he gets it — but can’t laugh for fear of driving off the road. “Of course,” Brennan says. “Safety first!” Cue adorable, knowing glances.