What is the test for evidence that qualifies as the "fruit of the poisonous tree"?

We explained that evidence qualifies as the "fruit of the poisonous tree" when "the illegal activity tends to significantly direct the investigation to the evidence in question." Id. (quoting United States v. Chamberlin, 644 F.2d 1262, 1269 (9th Cir.1980) ). "The focus," in other words, "is on the causal connection between the illegality and the evidence." Id . (citation omitted). Because "[t]he illegal stop was the impetus for the chain of events leading to the marijuana," the marijuana evidence was inadmissible. Id . at 24546. We also noted in Johns that "the burden of showing admissibility rests on the prosecution." Id . at 245 (quoting Chamberlin , 644 F.2d at 1269 ).