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 of the book 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 the cliff, his head had come off, so SpongeBob finds his "head" and puts it back on, but to their surprise, it was not his real head. He had become incredibly intelligent.