A what?
Once, long ago, this is what racing cyclists wore on their heads to ward off evil spirits (if they didn't wear a cap and a cabbage leaf as well, anyway). Now they wear plastic and polystyrene foam. Which is weirder?
They were light and cool. They fastened with a simple strap and buckle. They did little to protect the head from anything other than minor object impact. But it's what we had, at least until the "esky" was invented, circa 1980.
The esky was a bulky, hot and heavy alternative that provided much better impact protection but suffered from being, umm, hot and heavy. It was avoided as much as possible - even scorned - at least until the design improved and it evolved into the lightweight, cooler and more effective helmets we wear today.