The Physics of Fun is a dark chartreuse book with the text written in a faded brown color. The book seems to be very thick. As the title says, the book is about the physics of having fun.
Role in episode
Patrick reads this book to find out what has changed since he fell off a cliff in Jellyfish Fields. When he fell off said cliff, his head came off, so SpongeBob finds his "head" and puts it back on, but to their surprise, it is not his real head. He had become incredibly intelligent.