To recalcitrant plumbing ... and maybe other things

Third start - think I should give up. Was thinking about sink traps the other day. What the heck are they good for anyway? Thank heaven for Google. I found out. The water stops the nasty gas breaking into your home through the pipes. Hmm reminds me of some very spooky episodes of X Files. What I haven't figured out is why the trapped water smells so much. MOTH has to keep putting cleaner through the pipes to stop the smell. Something I have only noticed in the low set houses I have lived in. The ones built with a cement slab for foundations. How come it does that? Google hasn't answered that one.....