A childhood belief about imaginary friends

When our oldest boy was 5, we bought him a bow-tie, in a brown paper bag, for his first school concert, but his little brother cried because he'd missed out on a present.
In a moment of desperation, we told him, "There's no need to cry, your present is still in the bag."
Of course, the bag was empty, but we told him that only Mommies and Daddies could see the little Fresh-Air inside.
Well, we had that little Fresh-Air doing somersaults, jumping in and out of the bag and all sorts of circus tricks.
He carried that crumpled little brown paper bag around for months, showing it to envious friends and to grown-ups with looks of barely strangled laughter on their faces. We had to warn family in advance that the Fresh-Air was being brought for a visit.
Things came to a head 6 months later when he realised that nobody was feeding the Fresh-Air.
We told him, "He's eating the inside of the paper bag"...
"But there's no holes in the bag", said he, triumphantly...
"But it only eats the inside of the bag", we shot back...
That bought us another couple of weeks. He swears blind now that he twigged it straightaway.
We all know different...