What trick does oberon play on titania?

In William Shakespeare's play "A Midsummer Night's Dream," Oberon, the King of the Fairies, plays a trick on Titania, the Queen of the Fairies. Oberon squeezes the juice of a love-in-idleness flower onto Titania's eyelids while she is sleeping, making her fall in love with the first person she sees upon waking. He does this to punish her for refusing to give him her changeling child.