“I had a bag of muffins that had been in the cupboard for a little too long.” Instead of throwing them away, David Peasley of Alamosa, Colo., let the mold grow and spread before snapping this shot and submitting it to Scientific American. The fungus was probably a species of Penicillium, according to Kathie Hodge, a mycologist at Cornell University. “We can't tell for sure,” she says, “but the blue-green colony with white margin and the tiny pom-poms are consistent with an ID of Penicillium, especially if it's on bread.” In addition to bread, Penicillium is commonly found indoors on carpets and wallpaper. Some species can cause skin allergies and even death, whereas others are used to make cheeses and penicillin.

SUBMIT PHOTOS AT ScientificAmerican.com/what-is-it