"How many times are you going to rewrite the fractions lesson?"

If I 'teach' the lesson, not very often.

If I want each and every student to understand the concept of proportion, and each student be able to use fractions and ratios effectively, I'll need to adjust the presentation, sequence, style and approach a lot of times, quite possibly on the fly in the classroom.

Now, how many programming languages and code libraries do we really need?