Personal experience can sometimes illustrate the difference between the rational and irrational mind.  The somewhat superfluous rear windshield wipe on my vehicle used functions correctly only at rare intervals.  Normally, it would give the rear window one semi-effectual swipe and then get stuck, which helpfully also blocked the trunk from being opened.  When it did work correctly, however, I was not pleased, I was bothered.  To the irrational mind, those rare occurrences of proper function may have been pleasing-an act of divine providence or good luck-because their wish to be able to see out the back end of their car was fulfilled.  Hallelujah!  To my mind, having a windshield wiper that did not behave in a predictable manner is troubling.  My model of wiper operation was missing something.

What does a scientist do in this situation like this?  An experiment.  I started with a question:

Q: Why is the rear wiper getting stuck most of the time, but not all the time?

Then I thought about reasonable mechanisms that might cause this behavior to provide a framework for thinking about hypotheses.

Introduction: Since the wiper works sometimes, the cause is unlikely to be a flaw in the wiper mechanism.  Getting stuck can be caused by friction.  If different weather conditions change the friction between the wiper blade and the window, then the odds of getting stuck may change.  If the wiper blade is in bad condition, it might generate even more friction leading to the high probability of stuckageness that is currently occurring.

From this thinking, I developed testable null and alternate hypotheses:

H0: The wiper blade is not the source of the malfunction.
H1: The wiper blade is the source of the malfunction.

I then designed and constructed an experiment to distinguish between these hypotheses:

Experiment: Replace wiper blade.

The results of the experiment were as follows:

Results: Rear wiper has not become stuck since replacement of wiper blade. 

From which I could draw the following conclusions"

Conclusion: This supports the alternative hypothesis that the wiper blade was the source of the malfunction.  As a negative control (sham replacement of wiper blade-removal of blade with replacement with the same, original blade) was not possible due to sample size and safety restrictions, this conclusion must be provisional.

  Goodbye sense of unease.