Why can't I use the boolean modifier so, I'm really new to bender and I'm trying to make a ring for myself. but for this I have to remove part of the skull, but when I try to use the boolean difference modifier it simply doesn't change anything. But it doesn't act like it didn't work.

I want to cut the Cylinder from the sckull


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The topology of your skull is bad, there are vertices overlaying and inner faces, f you select some external vertices in Edit mode then press H to hide them and see what happens inside you will see it by yourself. Boolean doesn't work with overlaying vertices and inner faces, and I don't know if there's a quick way to fix the problem as there are a lot of vertices.