I have noticed that when I have lemon juice on my fingers and I touch a polystyrene foam cup (I will refrain from using the term 'styrofoam'), the cup begins to break down, i.e., melt. I can leave fingerprints on the cup in this manner. What is it about lemon juice and foam cups? What is happening, other than what I see; the cup melting.
I have never tried replicating this using other citric acids, such as an orange. So, I don't know yet if it's just lemon juice that will do this.