One of my kids told me this today, followed up with a "right mom?"
Now, I honestly don't know his feeling on the matter. He might have been joking, or saying something for the benefit of his younger sister.
Or, most likely, he knows.
I love the idea of knowing that something is real, and fitting it into the reality around you. It's such an innocent idea that we lose (maybe) somewhere on the road to adulthood.
The only reason why we don't see unicorns at pet stores is because that would be impractical. Unicorns are just too big.
It reminds me of the children's story, Many Moons, by James Thurber and Louis Slobodkin. The only thing that will save a girl from illness is for her father to capture the moon and place it on a necklace for the girl to wear (she knows that the moon is the size of the tip of her finger). Her father, the king, finally gets someone to make a tiny replica of the moon and threads it on a string, and then goes to great lengths to hide the real moon from her daughter. If she sees it, she'll know that the one around her neck is fake, and fall sick again. But, she does see the real moon hanging in the sky, and all is well, since the little girl knows that the moon grows back.
Fitting made-up ideas and people into the fabric of our understanding of reality is a lot of what fiction writing is all about. Listening to children might help us do it with grace.
I hope to have some of this perspective in my life, and in my writing.