I’d just like to add a bit of information to this classic Straight Dope column.
I bought a Hula Hoop for my young daughter last summer that contained a small amount of water inside the hoop. I’d estimate about 100-150 ml or so.
I didn’t compare the “water” hoop with a “dry” hoop, but I could feel the effects of the water. It gave the hoop a little more zip than what I remember the “dry” hoops had, which allowed the user (me , when I could wrest it from the little Spiffette) to whip it around with a little bit more Hula-Hooped fury.