system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are some skills "use them or lose them" but others are kept for life?

How is it that I can ride a bike for the first time in 10 years with ease, but I can't math myself out of a paper bag, even though I completed multiple calculus classes in college?
gpt
These are stored as different types of memories in different areas of your brain.

Riding a bike is an example of a “procedural memory” which is essentially muscle memory that is largely done unconsciously and automatically by your brain. 

In contrast, how to solve higher level mathematics is a “declarative” memory, similar to memorizing a phone number. These are facts that can be “declared” that you have to consciously remember. In school, all you’ve been taught is to memorize math as a series of facts (memorize this pattern of integration, etc.) and once those facts stop being useful you won’t hold onto them in memory anymore.

If you made a career out of mathematics, though, it is probably less likely you would forget how to solve problems, even after taking a break from it. It is also a lot easier for you to learn calculus the second time around, which is probably the true purpose of learning calculus in school—not to hope that you memorize all the facts past graduation, but for you to learn the reasoning behind it.