There are moments when we have the sudden awareness that we actually "understand/know" something. 

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"I finally grasped the year of Smalltalk I had taken in college." -- Francis Townsend

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"When struggling with algebra I finally discovered that a/x=b/y could be stated as a*y = b*x and that it further could be stated a*y -b*x = 0   Algebra was never a puzzle after that moment."

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I knew all about EddyCurrent from my studies of Electrical Engineering. 

Years later, when prompted by a friend to pass a very strong magnet over a the face of a very large copper/silver boulder, I felt Eddy pulling back... then I truly understood. -- MikeWarot

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These moments build on each other. There's a later moment when you realize you don't understand again.

''An AhaMoment or EurekaMoment should leave you with something you can keep, even if you can't write it down.  Maybe an OhNoMoment should, too...''

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I have found that when I teach people how to write programs recursively, they often snap all at once from not being able to understand anything to not understanding why they ever thought it was difficult.  All I can figure is that in order to understand recursion, one must first understand recursion; after that, it's easy.   --AndrewKoenig