You might be under the impression that plants photosynthesize—using energy from the sun to turn carbon dioxide and water into delicious and nutritious sugar—and you’re mostly right. Even carnivorous plants like the Venus flytrap practice this process to some extent (though nutrient-poor environments and inefficiency often lead them to supplement their diets with something a little bloodier). But not all flora are capable of feeding off of the sun. Some long ago abandoned this ability, having evolved other ways of gathering nutrients.
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.