Final answer:
Annie Sullivan helps Helen Keller to understand that words correspond to objects through a significant interaction with water at a pump at the end of Act II.
Step-by-step explanation:
At the very end of Act II, Annie Sullivan manages to get Helen Keller to interact with her through a breakthrough moment involving the water pump. Helen has a realization that the manual signs Annie has been teaching her correspond to the actual objects in the world. Annie guides Helen's hand under the water pump and spells out W-A-T-E-R while the water pours over Helen's hands. This repeated tactile experience combined with the signing helps Helen make the conceptual connection between the word and the substance.