system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why do we associate citrus scents such as lemons and oranges with freshness?
gpt
The actual ELI5 version: citrus has "fresh smelling" chemicals in it. 

The slightly less 5-year-old version: The characteristic smells of citrus are primarily chemicals called terpenes, things like Limonene primarily, but also Terpinene, Geraniol, Myrcene, Cymene, Linalool and others. 

These terpenes are found in a TON of organic plant matter that we tend to think of as "fresh", like flowers and herbs and grass. Avid potheads will probably recognize these terpenes as the things that give their sticky icky that characteristic dank nug smell.

They also tend to be quite volatile and dissipate quickly, meaning that not-fresh things rarely have a large quantity of these terpenes. Of course there are tons of exceptions to this rule, but this in broad strokes is why we tend to associate citrus with freshness.