Jaralee Annice Metcalf is a teacher who was inspired by an experiment found in the children's area of the Hospital C.S. Mott in Michigan and wanted to replicate with her elementary school students. Jaralee asked her students to touch different slices of bread to study the effects of dirt on their hands on food.
The experiment is very simple. It consisted of carefully taking several slices of bread and storing them in transparent plastic bags. Some of them would be caught with tweezers, others would be touched with dirty hands, others with washed hands and others after touching the keyboard of one of the class Chromebooks.
After this, the bags (with their bread slices) are labelled and left several weeks hanging from the wall.
The effects began after 3 or 4 weeks (probably due to bread preservatives) and while the one that had no human contact is very well preserved, the rest shows mould spots in different degrees.
It is a "homemade" experiment but very effective to teach children the importance of washing their hands before eating.