"Dad! There are worms in my cereal," Rosenfeld said was her proclamation during breakfast on the first day back to school after winter break.
Her father, Rick Rosenfeld, initially didn't believe her but then saw it for himself.
"I thought she was imagining it. She went and spit out what she had, and showed me. And yes, it was (a bug). And then we found another, and another."
In all, the family said they found 72 creepy crawlies in that box of Kellogg's Frosted Mini-Wheats. Twenty-five of them were alive.
Grossed out but intrigued, Erin says she went to school and told her classmates about her morning visitors.
"They were like, 'Oh my gosh, that is disgusting,'" she recalled.
And thus, an idea for her science fair project was hatched.
The Rosenfelds say they then went through that newly-opened box of cereal, putting the Mini-Wheats in water so they could see what came up. They say they garnered the dozens of yet-unknown bugs.
Exhibit One for her project: live critters. Exhibit Two: A letter she wrote to the Kellogg Company:
"Dear Kellogg Company: My name is Erin, I am a third grader and I am writing from my dad's computer with his help. I found worms or insects in my cereal, I got freaked out when I saw them in my cereal and haven't eaten cereal since. I am doing a science fair project about this. I found 70 meal worms, more than 25 were alive. It was a completely new box, and not open at all. Do you know how they got in? Do you know about this problem? What are you doing about this? Please be honest and write back as soon as you can. Thank you, Erin."
While she waited for a response, Erin Rosenfeld watched over the still-unknown type of insects. Her dad found evidence elsewhere.
Several people have posted videos to YouTube, claiming they also found bugs in their cereal, including one man whose approach was not appropriate to show any third-grader. So angry about what he found in his Kellogg's Frosted Mini-Wheats, he posted a video set to music showing off the problem.
Erin Rosenfeld may not have been nearly as mad as the YouTube guy, but she says she did get steamed at Kellogg's response to her letter.
"They responded pretty quickly with what was obviously a form letter, specifically written for insects in their cereals, and basically denied responsibility," Rick Rosenfeld said.
In part, the letter said that "scientific evidence shows that insects can penetrate properly sealed boxes."
To the Rosenfelds, it implied the bugs came from within their home. Did they?
To get answers, NBC Chicago took the intruders to the Notebaert Nature Museum in Chicago, where resident "bug expert" Vincent Olivares dove into the sample.
"It’s either a red flour or confused flour beetle," he said, adding that the babies likely came from the cereal factory or warehouse.
"All it takes is one adult female to make her way into the place where the cereal was made, and she can lay an egg, or up to 500 eggs," he explained.
For its part, Kellogg's said it has strict controls for every step of the manufacturing process, from the delivery ingredients to distribution, but in spite of that, "products are sometimes exposed to insects."
The company also said privacy laws prevented the company from engaging Erin Rosenfeld in an online conversation due to her age.
As an olive branch, Kellegg's did send the family a $1 coupon for another cereal purchase, but Erin Rosenfeld isn't biting at it.
"I'm like, 'You think I'm eating your cereal again?'"