When Lily was about two she told me she wanted a song about "no". This was ten years ago, and I don't remember why she wanted this, but I made something up:

This is a song about no.
This is a song about no.
This is a song about no, no, no.
This is a song about no.

The song goes... No no no no no no no no no.
The song goes... No no no no no no no no no.
The song goes... No no no no no no no no no, no, no.
This is a song about no.

It's useful anytime the kids want a song about something I don't know a song about. For example, it often served as a song about "turtle". Of course the more syllables the subject has the harder it is to sing, but that just makes it more fun.

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I applied my nascent music writing skills, and tried to set it down in dots:

Julia adds:

Oh I remember why it was invented. This was when Lily was still sharing a bedroom with us (out of necessity), when she was around 20 months. She would demand songs about various things as part of going to sleep. You were asking her, "Do you want a song about frogs?" But that night she answered "no" to all suggestions. So you asked, "Do you want a song about no?"
"... Yes."
So you sang this in the dark.