Some say that this song is probably about Native Americans. They often put their babies in cradles hung from branches. When the wind blew, it rocked the cradles, sending the babies to sleep.
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To me, there is something weird about a nursery rhyme in the English language about Native American customs. I rather prefer to see this nursery rhyme as a hint to a mystery well known to our ancestors but long forgotten in our current day and age of modern technology and science.
To find out more about this mystery, you are kindly invited to
The Castle of our white knight and hero
Braveheart.