Human memory is quite awful. The typical half-life of information that we learn is absurdly short:
We can lose up to 50% of what we learn in just an hour, and up to 90% within just a few days.
But there are ways to counter this issue.
Firstly, we can truly pay attention to the information we make and associate it closely with things we already know.
Secondly, we can break up our learning into repetitions, spaced over time. Thus…spaced repetition.
This method helps to nudge the curve from what you see above, to something that has a much smoother gradient:
And if we keep up spaced repetition for long enough, the curve has such a shallow curve that it is, for all practical purposes, never nudging from 100%.
This is why you never forget how to ride a bike.