Mum heard that Greg was seen throwing stones at another child:
"Greg, what have you been up to this afternoon?" [What, you expect me to confess if you don't know?]
"What do you mean? Nothing." he says, with an innocent, puzzled, expression.
"Did you throw stones at that new girl?" [Hmm, maybe I can get away with this.]
"No", he answers incredulous that you would even suspect him.
"Well Mavis says you did."
"It had to be some other kid." [Why would she believe a neighbor over me?]
"She sounded positive that it was you, Greg."
"She's crazy, it wasn't me!"
During the first exchange, Mum is tempting Greg to lie, and when he does she has him cornered. Now it's showdown time. Does she have total confidence in Mavis? Now Mum is having some doubts and doesn't quite know how to proceed. Greg is being insistent, so maybe it was another kid. If she chooses to believe him, she'll have to apologize for not believing him in the beginning. If she doesn't believe him, she will have two strikes against him; lying and throwing rocks.
Since most kids lie to stay out of trouble, it's up to you to promote truth and honesty. Don't tempt them to tell more lies. You have to make it beneficial for incriminating himself, instead of letting him off the hook. Lay a foundation of truth telling and honesty for your whole family. This means you can't lie either. Watch your children for honesty, and reward them when they are. Keep talking about how valuable honesty is as well as a good reputation. Show them that honesty is great and that it will be rewarded.
If you do suspect them of lying or some other bad deed, don't lose your temper.
If you already know what they did, don't ask, "What did you do?" That's just tempting them unfairly. Tell them what you already know and what the consequences will be.
You can also ask him to tell you the truth. Here is an example that won't giver him the option to lie about it:
"Hi Greg, I was talking with Mavis this afternoon, and she told me about something that she saw. I would like to hear your version. Why don't you go away and think it over for 15 minutes. Don't forget how much we value honesty in our home."
This will give Greg a chance to think things over. He can dig a bigger hole for himself, or he can be honest and face the consequences. If he tells you the truth, praise him for his honesty. If he chooses to lie, even after thinking about it, then you must double his punishment. He has now both lied and thrown stones at someone, and there are consequences for both.
When things quiet down, sit down with Greg and talk about what feelings may have led up to the incident. Maybe he was angry, envious, or insecure. Tell him that those feelings are natural and okay to feel, but that still doesn't excuse behaving badly. Be patient with him. He won't be willing to talk with you until he knows that you aren't' going to get excited and yell at him.