Teacher Donna Gill Allen has an ingenious teaching hack that is going viral.

The aim? To teach kids the importance of washing their hands, wrote news.com.au.

"This is the grossest yet coolest experiment," she wrote on a Facebook post.

Ms Allen took three pieces of bread. The first piece she simply put into a ziplock bag. For the second she made a point of washing her hands before handling it into the ziplock bag. The third piece of bread was passed around her entire class before being put into the ziplock bag.

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To all my teacher friends this is the grossest yet coolest experiment. I did this while teaching about germs and how...

Posted by Donna Gill Allen on Sunday, 13 August 2017

The image Ms Allen posted to Facebook best reveals the results.

It's fair to say that after a couple of days, Ms Allen's students would have been suitably grossed out by the "dirty hands" piece of bread.

Ms Allen, a Health Occupation teacher at Cape Fear High School in North Carolina, said the results speak for themselves.

"Watch how the bread changes over time due to germs. It is so cool and a great way to teach the importance of hand washing," she wrote.

Ms Allen explained to the Scary Mommy website how she came up with the idea for her experiment. "My students are Certified Nursing Assistants when they graduate from my program. I teach them to take care of patients. Part of their education is to teach them to wash their hands. This experiment was perfect for teaching about germs."