Kryptonite is a fictional glowing green rock from the Superman stories that drains Superman's superpowers and makes him weak and sick. In the comics and movies, Superman can lift buildings and fly faster than jets, but when kryptonite gets near him, he becomes as helpless as an ordinary person.

The word has become a useful way to describe someone's particular weakness or the one thing that can defeat them. A basketball player who excels at every part of the game except free throws might say that free throws are her kryptonite. A straight-A student who struggles with public speaking might call presentations his kryptonite. When your friend says that chocolate chip cookies are her kryptonite, she means she can't resist them no matter how hard she tries.

People sometimes like calling something their kryptonite because it acknowledges a weakness without shame. Even Superman, a very powerful hero in the DC Comics universe, has kryptonite. Recognizing what weakens you can help you either protect yourself from it or work to overcome it.