system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 why do we say "5 in the morning" but then say "5 at night"? why not "5 at morning" or "5 in the night"?
gpt
Focus on the prefix **"ing"** here. 

Would you say 5 in the evening? Or 5 at evening?

You can see when we substitute the word "night" for a synonymous one that ends in "ing", we would use, "in the" as opposed to, "at"