Andha Naal

On the night of 11 October 1943, during World War II,[3] the Japanese bomb the Indian city of Madras (now Chennai). The next morning, Rajan, a radio engineer and communications researcher, is found murdered with his own hand gun in his house in Madras after his neighbour, Chinnaiah Pillai, having heard the gunshot, complains to the police. Purushothaman Naidu, a local police inspector, arrives at Rajan's house and starts investigating the murder. In the meantime, Crime Investigation Department (C.I.D.) Officer Sivanandam joins Naidu to help with the investigation. Naidu suggests that the killer could be a thief who killed Rajan for the money found at the crime scene. However, Sivanandam is unconvinced by Naidu's theory because the amount of money at the scene matches the withdrawal entry in the bank passbook found in the same room. Rajan was about to leave Madras in anticipation of the bombings.
The two policemen question five people in and around Rajan's house, most of whom are his family members or friends. The first person to be questioned is Rajan's wife Usha, who is unable to speak because of her grief. Sivanandam and Naidu feel embarrassed and are reluctant to question her further. They begin interrogating Pillai, who reported the murder. Pillai proposes that the killer is probably Pattabi, Rajan's younger brother, and recalls a confrontation between them: Pattabi asked for his share of the family property but Rajan refused his request, feeling that he and his wife would squander it. Pillai concludes that this may have prompted Pattabi to kill Rajan.
Sivanandam and Naidu decide to interrogate Pattabi, who feels remorse for Rajan's death. He admits that he did not treat his brother well, and failed to understand his good intentions. He recounts an incident in which his wife Hema fought with Rajan because he refused to apportion the property. Pattabi states that Hema could have killed Rajan for the money as she loses her sanity when overpowered by anger.
Sivanandam leaves Naidu to interrogate Hema. She is initially impudent and refuses to give a statement about the crime, but she later agrees when Sivanandam threatens to arrest her husband. She reveals Rajan's extramarital affair with a dancer named Ambujam, who is pregnant with his child. As Rajan treated the news with a reckless attitude, Hema suggests that Ambujam could have killed him. When questioned, Ambujam accuses Pillai of the murder, saying that he was her foster father and wanted her to stay away from Rajan after the three met during a picnic. As their relationship continued, Pillai became infuriated and wanted to end the affair.
Sivanandam asks Usha, who tells him how she and Rajan fell in love. Sivanandam tricks Usha using a leaky fountain pen in order to collect her fingerprints. That evening, he and Naidu meet all of the suspects at Rajan's house. Sivanandam carries out an exercise in which the suspectsâincluding Ushaâmust pretend he is Rajan and shoot him using revolvers loaded with fake bullets. All the suspects shoot him except Usha, who bursts into tears. Sivanandam then pretends to have Pattabi and Hema arrested.
Unable to bear this, Usha reveals the truth: Rajan was a radio engineer who wanted to sell radios to the poor at an affordable price. Unable to get any support from the government, he went to Japan where his work was appreciated. He became a spy working for Japan, selling India's military secrets to the Japanese. Usha learnt about this and tried to reform him, but Rajan did not mind betraying India. Usha could not stop him and attempted to shoot him. She changed her mind but pulled the trigger accidentally, killing him. After revealing the truth, Usha commits suicide.

Question: What is leaking that allows fingerprints to be collected?
Answer: fountain pen