1 Paris: How did you become a genetic engineer?
2 fforbes-Davïs: I did a Ph.D. and post-doc studying gene sequences of the arugula salad herb species Diplotaxis and Eruca, then switched to humans.
3 Paris: That long on just a couple of plant species?
4 fforbes-Davïs: Well, it is rocket science.

Arugula.

To get this one you have to be cosmopolitan enough to know both the common American English and British English terms for this plant.

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2016-11-26 Rerun commentary: It's really frustrating when I come up with a hilarious joke that requires an in-depth knowledge of both cricket and baseball to get. This has actually happened to me a couple of times. It really doesn't help that very few of my friends are even sports fans at all, let alone of sports from two different continents. I do know one guy who'd get these jokes - but unfortunately he now lives in New York City and I've seen him exactly once in the past 15 years.