How to separate two adjacent faces?

I just knifed a simple mouth opening into a mesh, but I have no idea how to separate the faces from each other:

bild

I tried both Split and Separate with the above selection, but all that does is to create a duplicate of the highlighted edge.

With those edges selected do a Mesh -> Split -> Faces by Edges.

Thank you! I wonder why the right-click Split option doesn’t have that sub-menu from the mesh menu one…

The Split in the right-click menu is the Mesh -> Split -> Selection. Poorly named, poorly placed.

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are you looking for V to rip?

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Wow, yes, I see it now, thank you.

EDIT: Speaking of poorly named, what does “edge split” do compared to the other split in the right-click menu (and compared to the split in the mesh menu)? Also, someone told me that the shortcut is V, but that doesn’t even appear in the Mesh split menu!