What causes Puck to think that Lysander is the Athenian youth whose eyes he is to anoint?
Oberon wants to steal a changeling child from Titania, so he tells Puck to bring him a special flower. When the juice of the flower is dropped into someone's eyes, that person will instantly fall in love with the first person he or she sees. Oberon wants Titania to fall madly in love with him so that she will be unable to refuse anything he asks. Oberon has also overheard a conversation between Demetrius and Helena. She loves him, but Demetrius doesn't love her. So Oberon tells Puck to put a drop of the juice into Demetrius's eye, and describes him as the man wearing Athenian clothing, probably meaning that he is dressed in long robes or a toga. When Puck sees a similarly dressed Lysander, he mistakes him for Demetrius and puts the flower juice in his eye.