What property of styrofoam creates that awful squeaking sound when it is rubbed? See title. I will accept closure as duplicate of this question, but intuitively (as far as one can trust intuition in these matters) it seems like something different is occurring in the plastic/plastic materials than in the flesh/fluid materials, if only because the frequency and amplitude of the produced noise are so much more intolerable, and also the styrofoam degrades as you 'rub' it.
 A: The molecular structure of Styrofoam gives it a relatively high-friction surface, causing "stick-slip" to occur at the interface between your hand and the polymer.
Upon first contact, your hand "sticks" due to friction. In the brief moment(s) that your hand has not yet met the threshold force parallel to the surface required to overcome static adhesion, the walls of the polymer are compressed. When your hand finally "slips", the walls of the polymer re-expand. This process is repeated, and the frequencies of compression/expansion (in addition to other variables such as bubble size, density, and the gas used for extrusion (typically air)) gives rise to the audible frequencies observed.
A sufficiently large piece of Styrofoam will act as a resonator. Oscillations near the interface will cause oscillations in neighboring bubbles within the material, the end result being a loud "squeak".
There's also a biological component. The ear's frequency response peaks at $\sim 2 - 4 \text{kHz}$, at a gain of $\sim 10\text{dB}$. Higher frequencies, such as that produced by rubbing Styrofoam, experience greater amplification than lower frequencies.
A somewhat similar phenomenon is seen in "friction drums". You might imagine a piece of Styrofoam as a dense matrix of tiny friction drums.
See also: How is the squeaking sound produced, if you rub a finger over a wet, smooth, solid surface?.
