In his Treatise on Painting, Leonardo da Vinci suggested you could find new ideas by looking at stains on a wall. “Although it seems of little import and good for a laugh,” he wrote, the practice “is nonetheless, of great utility in bringing out the creativity in some of these inventions.”

By looking attentively at old and smeared walls, or stones and veined marble of various colours, you may fancy that you see in them several compositions, landscapes, battles, figures in quick motion, strange countenances, and dresses, with an infinity of other objects. By these confused lines the inventive genius is excited to new exertions.

I used to set down tea bags on an index card, and turn the stain into a drawing:

Leonardo notes that Botticelli used to throw a sponge with wet paint against a wall and find a landscape inside it. This is, essentially, how Ralph Steadman starts his drawings: first, a splash of ink, then seeing what the splash of ink wants to become…