This almost never works in practice. But here's the theory:
Open up your front door, and look at the latch (the piece that moves when you turn the doorknob). Notice that there are two pieces of the latch - the big piece that's angled or rounded off (the latch), and a smaller half-cylinder or button kind of piece that's right behind it. That smaller piece is called the deadlatch.
If you push on the latch, you can move it independently of anything else. That's what lets you close the door without turning the doorknob. But you'll also notice the deadlatch moves along with the main latch. Now, let go of the latch entirely, and just push in the deadlatch. You should find that you can't push the latch in if the deadlatch is already pushed in.
What happens when you close your door is that the latch and
the deadlatch both get pushed open together, but when the door is all the way closed, only the latch has room to spring into the recess in your door frame. The deadlatch stays pushed down, and now the latch can't move until the doorknob is turned. Under normal circumstances, that's what stops the credit card trick from working.
If for some reason, the recess of the door frame is way too big, and both the latch and deadlatch will fit into it, then you'd be able to push a credit card or something in between the door frame and the door, and use it to push the latch in (thereby opening the door).
But in reality, that almost never happens. In just about all cases, the deadlatch works like it's supposed to, and the credit card trick simply does not work.
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