The definition of the word phenomenon has some variants but all of them derive in the fact that it is an event that is observed as it happens and which explanation or origin is in question for the majority, mostly a phenomenon is understand or explain through the senses instead of the logical reasons before further investigation.
The body gives reasons because it is where you give evidence and reasoning. The introduction just introduces a topic while a the thesis does the same. The conclusion ends the writing so the only logical answer is the body.
