Many of my students arrive in college believing that lines are (in some way) made out of points.
They also believe that points have no length.
They want to know how a bunch of zero length points make a line of length 1?
Edit I did ask this question in math.stackexchange, but because it is about teaching it was declared off topic and closed. I changed to question there in hopes that if I understood it better, then I might be able to explain it.
I have asked my original question here in mathematics education in hopes of hearing teaching advice. I see it as a question about geometry and teaching geometry although it comes up in a class on ideas of infinity.