Greta Thunberg (Interview: Simon Hattenstone/Photo: Marcus Ohlsson)For the first ever cover of the new Saturday Magazine, Greta Thunberg made the ultimate sacrifice for The Guardian. She allowed us to turn her into a human oil spillage. The treacly black stuff was dripping from her hair, down her nose, past her cheeks on to her neck and shoulders. After telling Simon Hattenstone the paint was very difficult to get off he suggested that she sue the Guardian. Yes, she says without smiling. Reflecting on the interview,