Theory:

Because one day somebody oiled
His little whirling motor
And all the mystery was spoiled —
He ran as still as water.
Explanation:
 
The speaker gets curious about the sound the fan makes. He is pretty sure that the fan also has feelings and that it is trying to express itself through the squeaky sound that it makes. As long as it made the noise, the speaker felt like he had company. He did not feel alone or bored as he was trying to decipher what the fan was saying, But the speaker sadly recollects that one fine day the chatter of the fan came to an end.
 
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Motor oil
 
The speaker talks about how human beings eliminate the little mysteries and fun in the world. The fan kept squeaking in its own way. When the speaker thought of it as a beautiful way of communication, people around thought of it as a nuisance. They found a way to put an end to the fan trying to express itself. They were not able to view the situation from the same perspective as the speaker. They oiled the motor of the fan to reduce the noise. When a fan wears out due to constant use, it may create noise. Applying oil in the motor can ensure that the fan runs smoothly without any creaking. It is very similar to applying oil in doors to stop creaking. One has to remove the screws and blades before oiling the motor. Some fans have an oil hole and do not require the removal of blades. The speaker says that the mystery was spoiled because of the oiling. The fan no longer made any noise and the thrill that it gave to wonder about the fan's feeling was lost. The speaker says that the fan ran very smoothly as water. Water, when placed inside a bowl, moves in a very peaceful way without much noise until it is disturbed. Similarly, the fan has only slow movement but no noise.
 
Words with difficult meaning:
 
S.No
Words
Meaning
1
WhirlingTo move in quick circles
2
StillNot moving fast
3
SqueakingA short noise that is not too loud
4
DecipherTo understand something that is not clear
5
NuisanceSomething that is annoying
6
PerspectiveThe way one looks at something
7
CreakingThe noise of wood bending
Reference:
National Council of Educational Research and Training (2007). Honeycomb. Mystery of the Talking Fan: Maude Rubin (pp. 97-98 ). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.