Same Origin Policy in Layman Terms
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Can someone help me to better understand the Same Origin Policy. I've seen several websites describing it but I'm looking for an explanation much more simple, how would you describe it to a kid?

This <a href="https://developer.mozilla.org/En/Same_origin_policy_for_JavaScript">link</a>  seems to do the best job that I've found. Can anyone expand? Can someone explain why this policy exists?