Receipts and grease

How do receipts have that pencil like sketch quality when you scratch them? Is it some type of carbon-copy like coating? All paper should be like that so I don’t have to carry any writing utencils! Although it would be much more efficient to just carry a pencil versus a pad of magical drawing paper. Anyways, I came up with this quick sketch with my fingernail and in-n-out French fry grease. Yum.