The Orphanage

In 1975 Spain, a young girl named Laura is given up for adoption. Years later, adult Laura (Rueda) returns to the closed orphanage, accompanied by her husband, Carlos (Cayo), and their seven-year-old adopted son, SimÃ³n (PrÃ­ncep). She plans to reopen the orphanage as a facility for disabled children. SimÃ³n claims to see a boy named TomÃ¡s (Ãscar Casas). He befriends TomÃ¡s and draws pictures of him as a child wearing a sack mask. Social worker Benigna Escobedo (Montserrat Carulla) informs Laura that SimÃ³n's adoption file indicates that SimÃ³n is HIV positive. Incensed at Benigna's intrusion, Laura asks her to leave. She later finds Benigna in the orphanage's coal shed, but Benigna flees the scene. Later, SimÃ³n teaches Laura a game which grants its winner a wish. Clues lead the two to SimÃ³n's adoption file. SimÃ³n becomes angry, and says that his new friend told him that Laura is not his biological mother and that he is going to die soon.
During a party at the orphanage, Laura and SimÃ³n argue, and SimÃ³n hides from her. While looking for him, she encounters a child wearing a sack mask who shoves her into a bathroom and locks her inside. Escaping, she finds that SimÃ³n is missing and she is unable to find him. That night Laura hears several loud crashes. Police psychologist Pilar (Mabel Rivera) suggests to Laura and Carlos that Benigna may have abducted SimÃ³n.
Six months later, SimÃ³n is still missing. Searching in a Spanish city, Laura spots Benigna who is then struck and killed by a car. The police find evidence that Benigna once worked at the orphanage, and that she had a son named TomÃ¡s who was interned there. The children stole a mask that TomÃ¡s wore to conceal his deformed face. Embarrassed, TomÃ¡s hid in a local sea cave and was subsequently drowned. His death occurred weeks after Laura was adopted.
Laura asks for the assistance of a medium named Aurora (Geraldine Chaplin) in the search for SimÃ³n. Aurora conducts a seance during which she claims to see the ghosts of the orphans crying for help. Laura discovers the remains of the orphans she grew up with in the orphanage. They were killed with poisoned meals by TomÃ¡s' mother Benigna who wanted revenge on the children that were not blamed for the death of her son. She stored their remains in the orphanage's coal shed.[3] Unable to cope with the situation, Carlos leaves the orphanage.
Laura recreates the original orphanage layout and attempts to contact the children's spirits by playing a version of Peek Behind the Curtain. The spirits lead her to the door of a hidden underground room. Inside is SimÃ³n's corpse, wearing TomÃ¡s's mask. Laura finally realizes what happened: while searching for SimÃ³n the night he disappeared, Laura moved pieces of construction scaffolding, blocking the entrance to the secret room. The crashes that night were caused by SimÃ³n trying to get out. He fell and broke his neck.
Laura takes an overdose of sleeping pills. Dying, she begs to be with SimÃ³n again and the children's spirits appear, with SimÃ³n among them. SimÃ³n tells Laura that his wish was for her to stay and take care of the orphans, she then happily tells them a story. Sometime later, Carlos visits a memorial to Laura, SimÃ³n and the orphans. Carlos returns to the orphans' old bedroom and finds a medallion that he had given to Laura. He turns to look as the door opens, and he smiles.

Question: What method does Benigna use to kill the orphans?
Answer: Poison