Law courts are important institutional settings in which public understanding of science is problematic. Courts have struggled with the question of how to handle scientific evidence in a system of justice in which lay jurors are responsible for deciding the facts of the case. Judicial conceptions of science and of jurors' capacities to understand scientific evidence inform decisions in particular cases. Such decisions, in turn, act as precedents that, for better or worse, embed judicial conceptions of public understanding of science into the workings of the legal system. This paper examines an English case in which the difference between 'scientific' and 'common sense' evidence was explicitly at stake. In this case Regina v Adams, DNA evidence was used to convict the defendant of rape. The principal item of prosecutorial evidence was a match between DNA profiles developed from the defendant's blood and from semen recovered at the crime scene. Prosecution experts expressed the evidence in probabilistic terms, estimating a probability of one in 200 million that a DNA profile from a randomly chosen unrelated man in the relevant population would match the profile developed from the crime stain. Other so-called 'common sense' evidence supported the defendant's not guilty plea. The case was appealed twice in the mid-1990s as the Court of Appeal deliberated over an innovative effort by the defense to counteract the prosecution evidence by converting the non-DNA 'common sense' evidence into probability estimates. In its decisions, the Court reinstated a boundary between the 'scientific' and 'common sense' evidence, arguing that this boundary was necessary to preserve the jury's role as trier of fact. The paper's discussion of the court's boundary work addresses unresolved problems with the place of probability estimates in jury trials.