The judge will instruct the jury they are free to put what ever weight they believe on each and every witness. They are free to believe *some* testimony from one witness, while not believing other statements they have made, they are also free to judge 'if they believe a witness is lying about one thing, that all of their testimony is tainted.' They jury is instructed they are free to use their own common sense when deliberating. They are instructed on the definition of 'reasonable doubt'. Sorry, I can't recall all the standard jury instructions, but many of us have heard them many many times in the course of trail viewing, for me, over 20 years. The judge will also instruct the jury that 'opening statements and closing arguments are NOT evidence' and 'if you recall something different from what the attorney is saying, you are to go with *your* recall of the testimony.