example: I ate watermelon and crackers. No comma is used.
example: Glen owns a pick-up and a motorcycle.
example: My brother likes math and weird jokes.
example: My brother is neither friendly nor forgiving.
example: My brother is a math whiz but likes word games too.
listing three or more items, use a comma before "and" before the last item
examples The flag is red, white, and blue. Notice the comma use.
example: I ate watermelon, cantaloupe, and pretzels.
example: I will either eat pretzels, cookies, or brownies.
example of more than three: I ate watermelon, kiwi, strawberries, and blueberries.
example of paired items: For my snacks this weekend I ate watermelon and kiwi, licorice and peanuts, and celery and peanut butter.
In this example, items were paired up like a set. That is seen as a unit. So when you have three sets of items, follow the list of three rule (or the list of two if only two sets).
click here to play Rags to Riches & review commas in a series rule