I was on active duty in the Navy in Rota, Spain in the early 1970's. When you got there and went through the check-in process, one stop, of course, was at the dental clinic. They wouldn't immediately sign you up for flossing class. Yes; I'm serious and this would only happen in the military. Before they would make your first dental appointment, you had to have attended flossing class. I don't remember how long it lasted, probably somewhere from a couple of hours to a full morning. You got the theory of flossing, watched a dentist floss some fake teeth and then had ample time to practice on your own teeth. At the end you had to use disclosing tablets which would clearly show up areas you had missed with the floss and then you could go back and refloss them. I thought it was really stupid at the time. I was in my mid-20's back then, will be 70 next week and am still flossing pretty regularly. So, stupid or not, it apparently stuck.