In the ninth level of Chapter 2 "Beauty and the Electric Tentacle", a Beauty Goo is stuck on a large Goo line. If you can make the Beauty Goo roll backwards off of the line, she will fall down an invisible pathway with very strong airflow and explode in an invisible tube. The Goos will fall down onto ground below the boundary line, and a torch-lit wall can be seen with a mysterious drawing of a Goo ball connected in a bond by three lines and two formulas written near it. A sign can also be found below that reads:

Spoiler:"It's so simple." - the original Sign Painter

The formulas on the wall appear to be for the force of Gravity (Fg = m*g) and the Spring force (Fs=k*x) of one Goo bond. Assuming there is a damping term (Fd = d*v), the system the elastic physics of one Goo bond operates under would be a damped simple harmonic oscillator.

There is an achievement for finding this easter egg in the Steam and Games for Windows – Live versions of the game called "Subversive Traveler".
Contributed by MehDeletingLater