You might think that a word is 'easy' because you understand it. But do you really know all the uses of that word?
Take the word 'interest' for example. When teaching a class of advanced-level students recently, I used the phrase "out of interest" (e.g. I decided to take the course out of interest in the subject*). My students admitted that they had never heard the phrase "out of interest" before.
*out of interest in the subject = because I'm interested in the subject
If you check in a dictionary (click here for example), you'll see a lot more uses of the word 'interest'. Look through the list, and see if you learn anything new.