Smoking Guns In Films

What if you had that one piece of of information, that one key piece of evidence that solves everything right in front of your eyes? Well that would be what we're talking about today. The term smoking gun usually gets used in mystery films as the one single thing that helps the protagonist and the viewer solve a case but what are some great examples of this and how its used? As always we would love to hear your input on this and listen in as we crack this case wide open with "smoking guns in film".