**The Effect of a Thunderstorm on Milk: From Active to Passive Voice**

*Introduction*
When discussing the impact of a thunderstorm on milk, the usual statement is in active voice: "A thunderstorm often turns milk sour." However, we can transform this sentence into passive voice to shift the focus onto the milk rather than the thunderstorm. Let's explore this transformation in detail.

**Explanation in Passive Voice**
Milk is often turned sour by a thunderstorm.

1. **Subject-Verb Agreement**
In the active voice sentence, the subject is "a thunderstorm," and the verb is "turns." To convert it into passive voice, we need to make the object of the active sentence (milk) the subject of the passive sentence. Therefore, "milk" becomes the subject of the passive sentence, and the verb changes to "is turned."

2. **Passive Verb Form**
In the active voice sentence, the verb "turns" is in the present tense. In the passive voice, we use the appropriate form of the verb "to be" (is, am, are) followed by the past participle of the main verb, which is "turned" in this case.

3. **Indirect Agent**
In the passive voice sentence, any agent performing the action becomes optional or may be omitted entirely. If we choose to include the agent, it will be introduced by the preposition "by." In this case, the agent is "a thunderstorm." Therefore, the passive voice sentence becomes "Milk is often turned sour by a thunderstorm."

**Conclusion**
By transforming the active voice sentence "A thunderstorm often turns milk sour" into passive voice, we can emphasize the milk as the subject and highlight the effect of the thunderstorm. The sentence "Milk is often turned sour by a thunderstorm" accomplishes this by using the appropriate subject-verb agreement, passive verb form, and optional inclusion of the agent.

Milk is often turned sour by a thunderstorm