The "by zombies" test works because passive sentences allow a prepositional phrase starting with "by" to reveal the hidden actor performing the action.

This phrasing acts as a non-apology by dropping the subject, allowing a speaker to acknowledge an error without explicitly admitting who caused it.

While active sentences follow a subject-verb-object structure, the passive voice promotes the target of the action to the front of the sentence.

Passive constructions rely on past participle verbs like "eaten", tricking people into thinking phrases like "the cake is eaten" are in the past tense.