A request to the course makers...
I'd be so happy to see an "idioms and proverbs" bonus skill. Comment, all who agree! :-)
Initially I wanted to make a proverbs bonus skill. I was all geared up and went to the this wonderful Esperanto proverbs site. The problem was that every word taught requires three sentences and these proverbs often have really unusual word order. So, unfortunately, proverbs are out of the scope of a Duolingo course. As for idioms, you'll find them in the Esperanto Culture bonus skill when it's launched.
In any case, please visit the proverbs site I linked to above to dig deep into them. You can find a nice collection of proverbs at lernu too!