There is speculation on how director Zack Snyder will introduce kryptonite into the movie "Batman vs. Superman". I have a theory that may explain how it may happen based on my own thoughts. There is list of how it may happen. once again these are only my thought on how it could happen not proven facts .
- Batman may get a blood sample from superman and analyze it to find his weakness ( kryptonite ).
- Lex luthor may find Superman's weakness ( being his arch nemisis and all ) and share the news with the military and Wayne indusustries, thus giving Bruce wayne A.K.A. Batman the info.
Going with reason 1 right now would probably go down like this. When Batman finally confronts Superman , either in metropolis or Gotham , there may be a huge fight causing major damage. Before hand though batman did some research on the resident Kryptonian. Days before the fight he found a metal, the only metal capable of piercing Kryptonian skin . When he gets close enough to Superman he will puncture his skin and get the sample. Our Dark Knight will then analyze it back at the Bat-Cave or one of Wayne Industries labs. It will then be revealed... the Man of Steels weakness, kryptonite.
Reason number 2 although unlikely to happen is still a possibility to speculate on. Luthor ( played by Jessie Eisenburg ) being a mastermind is his specialty. he will most likely confront the man of steel and find his weakness or engineer kryptonite to stop or immobilize Superman. He will probably share with the military and/or government ... for a price.
Basically those are my theories. So i'll leave the rest up to you guys. you can post you theories on how it will happen or write your own article about it.