system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How is it that fiber can both help you with constipation and make your stool more solid?
gpt
The fiber cannot be fully broken down by your digestive system, so it basically just passes it on through your body whole.  This process helps to "push out" other waste material, and thus helps with constipation.  

Meanwhile you have these whole strands of fiber that are not broken down, that get mixed up with the rest of your waste material.  The end result is sort of like the rebar that runs through steel re-enforced concrete.