system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why can people without lisps easily imitate them without any training or effort, but people with lisps have a hard time speaking without them?

I understand there are a group of people with physical conditions that make speech production difficult (this question is not about them), but that most lisps are a result of the way a person has learned to shape their mouth .  

Why is this effect directional (easy one way, hard the other way)? Why can't people with a lisp imitate lisp-free speech as easily?
gpt
The same reason why a person with a good leg can easily imitate having a limp whereas a person with a lame leg can't easily imitate not having one.

There are many causes of having different kinds of lisps, but it is a kind of mistake (for whatever reason). It takes specific training and practice to correct it.