Child's Play

In 1988, Charles Lee Ray, a well-known serial killer and wanted fugitive, is seen running through the streets of Chicago. After he is fatally shot in a toy shop by Chicago homicide detective Mike Norris, Charles transfers his soul into one of the 'Good Guy' dolls via a voodoo spell. This causes the shop to explode, and Mike finds Charles's body.




Chris Sarandon played Detective Mike Norris.
The next day, a widow named Karen Barclay purchases the same doll (now known as Chucky) for her son Andy's sixth birthday from a homeless man. That night, Karen's co-worker and friend Maggie Peterson is killed when Chucky causes her to fall from the apartment window while she babysits Andy. Maggie had stopped Chucky from getting live updates on his ex-henchman Eddie Caputo, who abandoned Charles when he transferred his soul; she mistakenly thought Andy was disobeying her by not going to bed. As a result, the police search the apartment. Andy is deemed a suspect by Mike much to the annoyance of Karen, who orders Mike and the police to leave once they complete their investigation.
The next morning, Chucky orders Andy to skip school and take the train downtown. While Andy is urinating, Chucky sneaks into Eddie Caputo's lair, turning off a stove's pilot light but turning up the gas. Chucky toys with Eddie, who accidentally kills himself by shooting the stove, resulting in an explosion. Andy, once again a suspect, is placed in a mental hospital by Dr. Ardmore until further notice. That night, Karen discovers that Chucky's batteries were never inserted, and that Andy was telling the truth about Chucky functioning on his own power. While she is inspecting the doll, Chucky comes to life, bites her, abuses her and escapes. She then finds Mike at the station and shows him the scar that Chucky made. He does not believe her and leaves. After almost being killed by Chucky in his car, Mike finally agrees to help Karen.
Chucky goes to John, Charles Lee Ray's former voodoo teacher. When Chucky asks why he is able to bleed, John informs him that the longer his soul remains trapped within the doll, the more human the doll will become; in order to escape the doll's body, Chucky must possess the first person to be told of his possession with that being Andy. When John rejects Chucky's plea for help, Chucky fatally stabs John using his own voodoo doll. Chucky escapes just before Karen and Mike arrive on the scene. Before dying, John tells them that although Chucky is a doll, his heart is fully human and vulnerable to fatal injury.
At the mental hospital, Chucky steals the key to Andy's cell, but discovers Andy is not there. Dr. Ardmore finds Andy and unsuccessfully tries to sedate him. Chucky kills Dr. Ardmore with a violent electrocution and soon follows Andy home, knocking him unconscious with a baseball bat. As Chucky begins possessing Andy, Karen and Mike arrive and stop him. Chucky slashes Mike, then goes after Karen and Andy. The pair trap Chucky in the fireplace and burn him. Thinking Chucky is dead, Karen and Andy leave the room to help Mike, but Chucky follows them and attempts to kill them. Chucky is again thought to be killed when Karen shoots off his head, arm, and leg. Mike's partner Jack Santos arrives at the apartment, and disbelieves the trio's story just before Chucky's body then bursts through a ventilation duct and tries to strangle Jack. Mike then aims and shoots at Chucky's heart, finally killing him. Mike, Jack and Karen go to the hospital. Karen turns off the bedroom's lights and Andy looks back at Chucky's charcoaled remains before closing the door.

Question: What did Karen discover about Chucky that made her realize that Andy was telling the truth?
Answer: That Chucky batteries were never inserted.