Penal Code section 4502, prohibiting possession of a sharp instrument by a prison inmate, requires proof that the instrument is “sharp,” as the term “sharp” is commonly used. Appellant, an inmate in state prison, was charged with section 4502 after being found in possession of a hard plastic object, approximately three inches long, rounded on one end, and sharpened to a point on the other. Following presentation of evidence, the court instructed the jury that a sharp instrument is one that can be used to inflict injury but, otherwise, did not define “sharp.” In closing argument, the prosecutor urged the jury to follow this definition and added that “common-sense” definitions of “sharp” did not apply. Opining that the instruction removed from the jury’s consideration an element of the offense, i.e., that the weapon be “sharp,” and that the instruction was the focus of the prosecutor’s argument, the court reversed the conviction as it could not find the error to be harmless beyond a reasonable doubt.