Presenter notes: Open with a question that sounds obvious. Why is a mango sweet? Expect answers about sugar and about ripening. Then ask it properly: what does the MANGO TREE get out of making it sweet, since the tree never eats it? Silence, usually. Say that the answer is at the end of the lesson and it is worth waiting for, then move on. That unanswered question will keep at least half the class listening.