Running MongoDB Shell v-3.2.8

I've noticed that articles and tutorials always mention to run the `mongod` server before running the `mongo` shell. 

However, when I skip the first step and simply type `mongo` into my terminal, the mongo shell works without any errors / interruptions. 

```
MacBook:Desktop user$ mongo
MongoDB shell version: 3.2.8
connecting to: test

```

Why does this work? Does `mongo` call `mongod`?