Introducing Kryptonite

The League of Lois members use rifles that fire green energy bolts which can neutralize Superman. Although never named, it's obvious what their rifles are powered by. At the end of the episode, Mxyzptlk leaves Lois a piece of Kryptonite and deliberately chooses not to tell her what it is so that she'll find out for herself.

Example of:
Kryptonite Factor