The chemical reaction between lemon juice and polystyrene foam cups
It is not any acid in the juice, but limonene, the natural, fragrant, oil found in citrus fruit, and similar terpenes that are good organic solvents.

In fact, these oils are sold for the purpose of cleaning. Note, though limonene itself is considered comparatively nontoxic, after it has dissolved the polystyrene, it might be a slightly more dangerous, since any unreacted styrene monomer in solution is somewhat more toxic.