Image: AP Photo

No one saw it coming. It was, after all, just a simple plastic tube curved into a hoop. In the summer of 1958, the hula hoop boom began. At some point in their childhood — or adulthood — every American, from Boomer to beyond, has swiveled their hips inside a hula hoop.

As another summer arrives, undoubtedly countless hula hoops will continue to twirl on beaches and in backyards. That being said, the hula hoop almost disappeared as quickly as it blew up. Let's take a look at this simple-but-brilliant invention, as profiled in our June 9, 2017, episode of Through the Decades. 

Here are seven things you might not know about the hula hoop:

1) Withing four months of its introduction, 25 million hula hoops had been sold. 

2) The hoop was the brainstorm of Richard Knerr and Arthur Melin, co-founders of Wham-O, the same men who had previously scored a big hit with another invention — the Frisbee. 

3) Knerr and Melin were so unprepared for the hula hoop's success, they neglected to patent it, which allowed competing hoops to rise up, including one from TV personality Art Linkletter.

4) By 1960, sales inevitably tapered off. Just as Knerr and Melin had not anticipated the craze, they hadn't foreseen its end, leaving them marooned with a huge stock of unsold inventory.

5) After selling almost 10 million hula hoops, Knerr and Melin's total profit was just $10,000.

6) Nevertheless, the two still believed in the inherent appeal of their invention, and after finally securing a patent in 1963, Knee and Melin unleashed the noise-making "Shoop-Shoop Hula Hoop."

7) Originally, Wham-O experimented with walnut shells inside the hoops to make sound. The shells were later replaced by ball bearings, which did the trick. The Shoop Shoop Hula Hoop became a steady seller, a toy store evergreen, and every now and again became a minor fad again.

Learn more by watching Through the Decades on Decades, weekdays at 7am, 1pm, 7pm and 10pm Eastern time. Find out where to watch where you live.