Yes, yes, rat pee.

From the Guardian:

Gravett's computer skills are largely self-taught and she talks knowledgably about them. But after blinding me with science on the subject of electronic collage, she reveals that much of the work here was done by her daughter's pet rats - she needed a lot of nibbled effects, so she painted the relevant areas with yoghurt and the rats obliged. They also generously supplied urine samples which, in combination with more conventional watercolour techniques, have created some very authentic stains. Like all the best picture books, Little Mouse's Big Book of Fears invites and rewards endless exploration at all levels of understanding, and aside from rats' urine these pages are full of inventive, creative ideas for aspiring artists - look at the "fear of being eaten" page (Phagophobia) to see how feathers can be scanned in and graphically manipulated. Readers are encouraged to explore their own anxieties, and to add their own artwork - librarians had better watch out.

Earlier article about the urine!  Whee!