Answer:
What does Oberon do with flower juice? : Oberon creeps up on her and squeezes the flower's juice onto her eyelids, chanting a spell, so that Titania will fall in love with the first creature she sees upon waking. Oberon instructs Puck on how to use the juice of the flower 'love in idlesness' to get revenge on Titania and to make Demetrius fall in love with Helena.
Why? : Oberon sends Puck to find a special flower that, when it's juice is placed on the eyelids, causes that individual to fall in love with the next living creature seen. He plans to put it on Titania's eyelids to cause her to give him the boy. Helena and Demetrius come to the woods hunting for Hermia.
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