system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Do water pipes contain a nasty film inside of them?

ELI5: What stops the inside of pipes and water lines from getting coated in that nasty film that collects on containers that has water in them to long?
gpt
Usually it’s due to the constant supply of fresh water running through. Think of it like the difference between a river and a stagnant lake. Rocks in the river will be smooth and clean and the lake might be murky and sedimented. The small impurities we don’t otherwise notice will gather and settle.  If you have ever seen an old fire sprinkler line burst, you’ll have seen some of the blackest, nastiest, foul smelling water.