Each crime has its own set of elements—components that the prosecution must prove beyond a reasonable doubt in order to establish the defendant's guilt. If the prosecution fails to prove any element, then the jury must acquit.
For example, a drug possession law might require that the defendant:
So, if the prosecution can establish that Michael had a bag of marijuana in his pocket, but can't show that he knew that the substance was cannabis rather than a Caprese salad, the jury must acquit. Or, if the government can show that Wallace had cocaine inside his jacket pocket, but that it was only a trace amount that couldn't be used, then he isn't guilty.
For further reading on a related topic see Lesser Included Offenses.