How is a raven like a writing desk?

Knuckles is getting really, really frustrated at us because he keeps asking for "a sandwich like oxygen." Whatever that is, we're unable to give it to him, and every moment that we don't give it to him his frustration is building. When we ask him to show us "oxygen," he places his forefingers and thumbs together to make a diamond shape. We asked him if that meant he wanted his sandwich cut into triangles, or squares, but that wasn't it either.

What the hell is "a sandwich like oxygen," and why is it so vital to the mental health of a three year old boy?