Back at the lab, Walter is injecting some of the serum from Officer Gillespie's into the sliced-open end of a watermelon, much to Astrid's distress. Apparently she has a rule about experiments on fruit. "I just cleaned this lab up yesterday," she complains. Okay, that sounds like something to take up with the FBI chain of command, unless she went to Quantico to become a maid. He says he's only using a small amount, and as he gives her a pair of safety glasses to match his own, he says they know the bomb's active ingredient but they don't know the trigger. So he's testing Peter's theory that it might be radio waves, twiddling some knobs on a radio set. He explains that the same concept was used with proximity fuses in WWII aerial bombs. "In fact, my uncle Henry was killed by one in the battle of Guadalcanal," he adds, which I wouldn't bother to transcribe if I weren't 63% sure that it's going to be some kind of clue in the future. At this point, the lights in the lab are flickering and the watermelon is vibrating on its stand. Seeing the surface crusting over the way we saw with Captain Burgess's foot and Officer Gillespie's everything, Walter asks Astrid the frequency they're currently broadcasting on, which is 331.6 Mhz. Then there's a moment of calm, and they wonder why it stopped. Then the watermelon explodes. Judging by Walter's excited reaction, a similar event has occurred in his pants. "No more fruit, Walter," Astrid bosses. She doesn't seem very excited that they found the right frequency on the first try.