"As our writers and language experts develop lessons, they work on balancing really useful phrases that you can use immediately, like ¿Cómo se llama tu hijo? 'What is your son's name?' with the quirky and fun sentences that stick in your memory."
"We want our content to be as full of personality as we are! Our writers' first priority is always the lesson's learning objective, but after that, they are encouraged to develop humorous, interesting content, with some quirks mixed in."
And it appears to be working. According to a study of 225 Duolingo learners from the U.S. who had reached the end of Unit 5 (the beginner-level section of the course), they performed as well on reading and listening tests as students who had completed four semesters of university language instruction.
Another study looked at 340 learners from across the globe who had reached the end of Unit 7 (just halfway through the intermediate material of their course) and found that they completed reading and listening tests just as well as students who had gone through five semesters of university language instruction.
So kudos to silly sentences!
