"Needle in a haystack" means something that is tiny, lost or hidden, impossible or extremely difficult to find, especially because the area you have to search is too large.
The idiom is at least five hundred years old, with the oldest known usage being from an early English translation of the early 17th century Spanish novel Don Quixote de la Mancha by Miguel de Cervantes.
One example:
Finding the piece of paper I need in this huge pile of documents is like looking for a needle in a haystack.
From online with editing
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Still remember the time in my salad days, riding a bike running between libraries in Beijing, to find engineering journals for my thesis. With the wonderful internet search engines and help of AIs today, it's much easier to find "a needle in a haystack" for researchers and the like. 
But how about finding a piece of memory in your head? Your first day in school? Your first talk with your best friend? Your first crush (remember mine, but I 'm not going to tell 
). Past APAD words we discussed here (without searching)?
Although the ROM/RAM in a human's brain ain't as big, with limited brian CPU power, it may still feel like looking for "a needle in a haystack".
Or, the needle might not be in the haystack at all.
Have a good Monday
