I do believe in fairies.

My son, Aidan, walked into the living room, an envelope in one hand, a thick marker in the other. He sounded out each word as he wrote: “Contents: one tooth. Please leave $5 under the pillow.” He looked me in the eye and said, “Apparently, the tooth fairy hadn’t noticed my loss.”

My husband, Brian, paid no attention. Normally a helicopter parent, except for two weeks every four years, he sits when watching slaloms, quadruple salchows and half-pipes and whatever else passes for an Olympic sport. There’s some kind of event with brooms and a hockey puck, but I’ve never figured that out.

But Aidan made sure I noted the delinquency of the pixie who visits the outer, outer, outer Excelsior. And, this, this was the very last of the 40 milk teeth that will ever leave the Bedlam Bungalow.

The boys have maintained a great relationship with Santa Claus, who, despite the behaviors of all four of us, has never once left a lump of coal. The Easter Bunny is a little more tentative in that he keeps forgetting neither of the boys eat hard-boiled eggs, but he has started bringing hot Cheetos and sour gummies.

But this was the end of an eight-year relationship.

On June 6, 2010, Zane earned his first dollar. Ms. Francis, his first-grade teacher at Harvey Milk Civil Rights Academy, said 10 months earlier, “If any of you finish first grade with me in June without having lost a tooth, I will give you a dollar.” Twenty-two kids each with 20 choppers, her odds were roughly 440 to one that she would have to pay off. But Zane loved a long shot, and no matter how many candy apples and corn on the cob that kid went through, he made it with all 20 pearly whites intact.

That night, Maya, the girl-next-door, invited Zane and Aidan over for dinner, a celebration of the end of the school year. They watched “Tooth Fairy,” a film about a little girl who had lost her tooth, and a hockey player telling her she wasn’t getting anything for it, and how they both learned he was wrong.

I should have seen it coming, but as the credits rolled Zane asked, “Tell me the truth. Is there really a tooth fairy?”

Belief is such a fragile thing. Doubt is much easier, but once you start suspecting miracles, then nothing seems certain. This was the kid who had asked me, “Who’s faster? The Flash or God?” and I no longer had a neat little theological package for him.

So I took advantage of the boys’ ADHD, and suggested playing on the Wii, knowing that whatever else they questioned, their faith in Mario and Luigi was steadfast. I never did have to answer the question.

The next day, Zane dropped a bicuspid. Maybe it was Maya’s popcorn. Maybe it was a test of faith. But a dollar showed up under his pillow, and all was right with the world.

Aidan saw easy money. Started wiggling incisors, and by the next morning he had a buck.

He gave me a big hug, which I especially enjoyed, because he doesn’t like me quite as much as any of his uncles. From that day, he worked the chimerical personages who visited: “Daddy, do you think Santa knows I want an Xbox?”

“Daddy, I hope the Easter Bunny knows that I don’t like Peeps. They take up all the room, when I’m really just looking for chocolate.”

“Daddy, are you sure there’s no such thing as a birthday fairy?”

Aidan was just the opposite of me. Each visit to Dr. Moyer, I hoard my teeth, fearful that this was the time no root canal could save that molar from six decades of abuse. No, I treated each and every tooth like it literally had a gold crown. But Aidan kept ripping out premolars, sometimes by the root, and each time, he announced, “This one’s worth at least two bucks.” True story: He tried to pass off one of Krypto’s teeth as his own.

Watching a luge run down the track, I finally had the answer to Zane’s long-ago question: Doubt is easy, but when you get down to it, there’s no payoff in doubt. Once you no longer cherish mystery, you stop enjoying the moment. The real magic in the world comes from believing when there is no proof, allowing for sprites who pick up used teeth, trusting that there is such an improbable thing as love, and that families are made out of this love, no matter how strangely they are put together.

Aidan woke up to a five-dollar bill.