Children smudging their hands onto every available surface is normally gross and annoying — unless, that is, the available surface in question is a sterile plate, which you then incubate in a petri dish.
Wanna create your own beautiful, living, breathing solar system at home? It's easy—you just need petri plates (or any kind of flat, wide and shallow containers), some fatty milk, a little dish soap, food coloring and liquid plungers and droppers. And your own imagination.
Salmonella. Escherichia coli. These once feared bacterias have joined the forces of good to sanitize your hands as fancy soaps.