How does our brain know we forgot something, but can’t remember what it is?

Question

Sometimes it takes me to backtrack my thought process until I remember, sometimes it randomly comes back. Sometimes it doesn’t. Why?

In: Biology

Answer 1

This is a slightly above ELI5 answer (maybe more ELI12):

An average human has 86 billion neurons (brain cells), which between them form 100 trillion connections. Memory, and all other brain functions, exists as a complex interaction of billions of these neurons.

Let’s say you leave your car keys in a drawer instead of the table you normally put them on. Your “memory” of car keys involves what a car is, what a key is, what car those specific car keys go to, how a key works, what other keys are on that key ring, where those keys normally are, how those keys feel, how they smell, and dozens of other small connections between those keys and other memories/knowledge of them you have.

Sometimes a short term memory, like dropping your keys in a drawer, only make one or two connections. Sometimes those specific memories and connections don’t trigger until we have a strong enough stimuli to activate it. With short term memory that wasn’t deeply ingrained to have made a lot of connections, you have to reactivate stimuli strong enough to activate that one memory. So retracing your steps precisely might be that one connection you have left to that one specific memory.