Are the nouns in this sentence brains and watermelons? Their brains may be as big as watermelons.

Yes.

Yes, in the sentence "Their brains may be as big as watermelons," the nouns are "brains" and "watermelons."

To determine the nouns in a sentence, you can follow these steps:

Step 1: Identify the subject of the sentence. The subject usually tells you what or who the sentence is about. In this case, the subject is "their," which is referring to a group of people or things.

Step 2: Look for the words that follow the subject and describe or relate to it. In this sentence, "brains" is directly related to "their." It tells us what belongs to "their." "Watermelons" is used to compare the size of the brains, so it is also a noun in this sentence.

By analyzing the sentence and understanding the role of each word, you can identify the nouns in the sentence.