I’m one of those people with straight hair that can’t get curls or even waves by using a curling iron. It just doesn’t work. And even if it does make my hair slightly wavier (requiring massive amounts of hairspray), it doesn’t last for more than a couple hours before the waves go flat and straight again.
For years, the only way my hair could get any kind of lasting waviness was if I went to bed with my hair wet in actual curlers, but who wants to wear curlers to bed all the time? Definitely not me.