The Doctor Proctor’s Fart Powder series for kids, by Jo Nesbo. Currently, there are four books in the series. I read them in English translation by Aladdin Books, 2007-2012, illustrated by Mike Lowery. The series includes: Doctor Proctor’s Fart Powder Bubbles in the Bathtub (Doctor Proctor’s Time Traveling Bathtub) Who Cut the Cheese? (The End…