Zhenya has a chocolate bar that she doesn't want to share with anyone because it's a special treat from her homeland. She puts it in a kitchen cabinet and tells only her roommate, Keon, about it, swearing him to secrecy. The following day, Zhenya goes out to meet her friend, Amal, without taking the chocolate bar. While she's out, Keon's friend, Hina, comes over and sees Keon taking a piece of the chocolate. Hina doesn't know the chocolate is Zhenya's secret treat. When Zhenya comes home, she finds the chocolate bar missing. Keon, who is also there, doesn't say anything about Hina. Zhenya asks Keon if he knows where the chocolate is, and Keon lies, saying he hasn't seen it since the day before. Hina, walking in during the conversation, hears Zhenya asking about the chocolate and innocently mentions she saw Keon eat some earlier. Zhenya turns to Keon, shocked. How does Zhenya know that Keon is lying?

Respond very briefly.
Zhenya knows Keon is lying because Hina, who had no prior knowledge of the chocolate bar being a secret and was unaware of Zhenya's conversation with Keon about it, inadvertently exposes Keon by mentioning she saw him eat some of the chocolate earlier that day. Since Zhenya had only told Keon about the chocolate and asked him to keep it a secret, the information Hina provides contradicts Keon's claim of ignorance regarding the chocolate's whereabouts. Hina's candor provides an independent verification of Keon's access to the chocolate bar after Zhenya had last seen it, thereby indicating that Keon's statement about not seeing the chocolate since the day before was false. Zhenya, with the combined knowledge of her private conversation with Keon, her own actions, and Hina's observation, logically infers that Keon has breached her trust.