In the late 1960s, psychologist Walter Mischel performed a series of tests on preschoolers referred to as The Marshmallow Tests. Mischel would give a child a single marshmallow, then leave him or her alone in the room with it. Before he departed, he'd make each kid an offer: if they wanted to, they could eat it immediately -- but if they waited for him to return, they'd get two marshmallows. The tests were designed to examine willpower and the mental processes behind delayed gratification. Watching kids go through the experiment can be poignant... and adorable.
As a recent New Yorker article on "the secret of self-control" put it:
Footage of these experiments, which were conducted over several years, is poignant, as the kids struggle to delay gratification for just a little bit longer. Some cover their eyes with their hands or turn around so that they can't see the tray. Others start kicking the desk, or tug on their pigtails, or stroke the marshmallow as if it were a tiny stuffed animal.
Watch a reenactment of the tests: