Superman

On the planet Krypton, using evidence provided by scientist Jor-El, the Council sentences attempted insurrectionists General Zod, Ursa, and Non to the Phantom Zone, for which Zod swears revenge on Jor-El and his family. Jor-El, despite his eminence, is unable to convince the Council that Krypton will soon be destroyed when its red supergiant sun goes supernova. To save his infant son, Kal-El, Jor-El launches a spacecraft containing him toward Earth, a planet with a suitable atmosphere where Kal-El's dense molecular structure will give him superhuman powers. Shortly after the launch, Krypton's sun explodes, destroying the planet.
The ship crash lands on Earth near Smallville, Kansas. Kal-El, who is now three years old, is found by Jonathan and Martha Kent, who are astonished when he is able to lift their truck. They take him to their farm and raise him as their own, naming him Clark after Martha's maiden name.
At 18, soon after Jonathan's death due to a heart attack, Clark hears a psychic "call" and discovers a glowing crystal in the remains of his spacecraft. It compels him to travel to the Arctic, where the crystal builds the Fortress of Solitude. Inside, a holographic vision of Jor-El appears and explains Clark's origins, educating him on his powers and responsibilities. After 12 years of training, with his powers fully developed, he leaves the Fortress wearing a blue and red suit with the House of El family crest on his chest and becomes a reporter at the Daily Planet in Metropolis. He meets and develops an unrequited romantic attraction to coworker Lois Lane.
Lois becomes involved in a helicopter accident where conventional means of rescue are impossible, requiring Clark to use his powers in public for the first time to save her. That night, he thwarts a jewel thief attempting to scale a building using suction cups, captures robbers fleeing police, rescues a girl's cat from a tree, and rescues Air Force One after it loses an engine, making the mysterious "caped wonder" an instant celebrity. He visits Lois at her home the next night and takes her for a flight over the city, allowing her to interview him for an article in which she names him "Superman".
Meanwhile, criminal genius Lex Luthor has developed a plan to make a fortune in real estate by buying large amounts of barren desert land and then diverting a nuclear missile test flight to the San Andreas Fault. It will sink California and leave Luthor's desert as the new West Coast of the United States, greatly increasing its value. After his incompetent henchman, Otis, erroneously redirects the first rocket, Luthor's girlfriend, Eve Teschmacher, successfully changes the course of a second missile while the military is distracted by a roadblock Luthor arranges.
Knowing Superman could stop his plan, Luthor lures him to an underground lair and exposes him to Kryptonite. As Superman weakens, Luthor taunts him by revealing that the first missile is a decoy, headed east toward Hackensack, New Jersey, realizing that even Superman cannot stop both impacts. Teschmacher is horrified because her mother lives in Hackensack, but Luthor does not care and leaves Superman to a slow death. Knowing his reputation for keeping his word, Teschmacher rescues Superman on the condition that he will deal with the New Jersey missile first. After diverting the eastbound missile into outer space, the other one aimed at the West Coast explodes near the San Andreas Fault. He is able to mitigate the effects of the nuclear explosion, getting rid of the pollution from the fallout and shoring up the crumbling Earth, but the aftershocks are still devastating.
While Superman is busy saving others, Lois's car falls into a crevice that opens due to an aftershock. It quickly fills with dirt and debris and she suffocates to death. Angered at being unable to save her, Superman defies Jor-El's earlier warning to not manipulate human history, preferring to heed Jonathan's advice that he must be on Earth for "a reason". He accelerates around the Earth, rewinding time, in order to save Lois. He then delivers Luthor and Otis to prison and flies into the sunrise for further adventures.

Question: Why does Superman weakens?
Answer: He is exposed to Kryptonite.