TL;DR

Know how to see which device a file belongs to.

From time to time, I have to rediscover how to do this.

This being a very simple thing: find out which underlying device is actually holding a file or a directory. As an example, I sometimes have to check whether there is enough space in the current directory, and in case go somewhere else.

The bottom line is that df is my friend, and I hope I will remember it when I will need this bit of information the next time:

$ df "$HOME"
Filesystem                    1K-blocks     Used Available Use% Mounted on
/dev/mapper/foobarx--vg-root  36643428  15040292  19712016  44% /

This is it: just pass the path to the file/directory you want checked, and you will get back the only relevant line from df.

Brilliant!