How do kids stop believing in Santa Claus?

From a Slate article:

As Occidental College cognitive scientist Andrew Shtulman writes in a study soon to appear in the journal Cognitive Development, “Santa violates our expectations about spatiotemporal continuity by visiting all the world’s children in a single night; he violates expectations about containment by entering children’s houses through their narrow chimneys; and he violates expectations about support by flying through the air on a wooden sleigh.” Still, kids' belief in Santa is stronger than nearly any other fantasy character. . . .

Shtulman rounded up 47 children between the ages of 3 and 9. All the kids in the study said they believed in Santa, but it turned out that they didn't all think about Santa in the same way. An older child who was more capable of identifying the implausibility of Santa Claus would argue, for example, that Santa could know whether every child was naughty or nice because, as one reported, “He has cameras all around the world.” Or they might suggest that Santa's reindeer can't actually fly; they're held up by yarn. By contrast, younger children would simply answer that Santa's reindeer fly thanks to magic.

The children were attempting to reconcile the folklore surrounding Santa's superhuman abilities with their developing knowledge about the constraints of the physical reality in which they live. Some kids were already better at distinguishing the plausible from the impossible (for instance, when asked, they said that pickle-flavored ice cream is possible but unlikely, while applesauce can never be turned into an apple). These kids had also “begun to engage with the mythology surrounding Santa at a conceptual level, questioning the feasibility of Santa’s extraordinary activities while also positing provisional explanations for those activities in the absence of a known answer,” writes Shtulman.

Granted, Shtulman wasn't necessarily interested in the question of Santa per se. Nor could he directly assess the kids' skepticism, because provoking young kids to question the plausibility of Santa might draw the ire of their parents. Instead, he sees Santa-related lore as exactly the sort of false knowledge that's typically transmitted to kids from people they trust most: parents and teachers. “Studying children's beliefs about Santa can shed light on how children interpret testimony that they cannot personally verify through firsthand observation,” he says.

For most kids, at some point the weight of evidence against the likelihood of Santa being real becomes too heavy to sustain their belief, even if their parents continue to encourage it. Shtulman relates the story of one child whose mother continued to talk up Santa. Then, as she set about wrapping presents, she found a note that her son had written on the back of the paper: “If Santa uses this paper, Mom is Santa!”