Definition: In this task, you will use your knowledge about language (and common sense) to determine what element the marked number refers to. The numbers are marked with two underlines around them, like: _ number _. Your answer should be chosen from the given text, and should not contain other words.
Input: Ernest Pratt:  What the heck is that ?
Janos Bartok: Ball lightening .
Huitzilopochtli Ramos: Actually , Aristotle called them fire balls .
Janos Bartok: Thought of by many renounced men of science to be an optical illusion .
Ernest Pratt: Darn good _ one _ , if you ask me .
Janos Bartok: Yeah , I thought it was an optical illusion myself until I saw one appear accidentally by the tower . I determined that it was a resonance of the electromagnetic field . And what you are seeing , for the very first time in the history of man , is ball lightening created artificially .
Ernest Pratt: What can you do with it ?
Janos Bartok: I have no idea . It could change the course of human events . Possibly not . Right now it 's delightfully useless ; and we shall follows this useless research where ever it shall take us !

illusion