memory
In one ear and out the other
Quickly forgotten
Slip one´s mind
Forget to do something.
A bright spark
A very intelligent person.
All in your head
It is not real. It is your imagination.
Brainstorm
Come up with creative ideas.
Etched in (one´s) memory
A memory that is unforgettable.
Lose one´s train of thought
Forget what you were thinking.
Not the sharpest tool in the shed
Someone who is not very intelligent.
Can´t make heads or tails of it
You can´t understand it at all.
Rack one´s brain
Try very hard to remember or figure something out.
Ring a bell
Sound familiar but not completely remembered.
Out of sight, out of mind
If you don´t see it, you forget about it.
Street smart
Intelligent in a practical or survival sense.
Drum something into someone´s head
Teach something through constant repetition.
Brainchild
A creative idea or invention that someone came up with.
A trip down memory lane
Remembering the past nostalgically.
A senior moment
Forgetting something, usually as a joke about aging.
A few fries short of a Happy Meal
Not completely intelligent.
Off the top of your head
Give an immediate reaction, not a carefully considered opinion, so it might not be correct.
No-brainer
A decision or solution that is very obvious.