Sky

Clouds in the sky very much resemble the thoughts in our minds! Both change perpetually from one second to another.

~ Mehmet Murat Ildan

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Both clouds and thoughts can block out clarity - of the sky, of the mind. Both can be striking when you shine the right kind of light on them. Both clouds and thoughts only have the vowels "o" and "u" in them. 

(pause)

This last observation might be slightly less interesting and might lack the depth of the others.