system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: Why does there have to be water in a toilet all the time?

Urinals have proven that there doesn’t need to be water all the time for number 1. So for number 2, couldn’t there only be water when the toilet flushes?
gpt
The water forms a seal that prevents gasses from the sewer from coming back into the house.  It’s the same reason there is a P-shaped bend in the pipe under your sink.  Water collects there and forms a seal.