Sometimes the problems you met, throws you multicore programming course at university.
Sometimes, you solved it before and do not remember how to solve it. Just like running the NoSQL query without an ID, remembering how it was solved after 5 days; It reminds me to wait for the migraine to pass without using painkillers.
According to books which tell about how brain works, our brains know but don't know what they know. Proving this is a kind of NP Problem, am I right?