on the brain - if you have something on the brain you never stop thinking or talking about it

in a positive frame of mind - happy and ready to go ahead and do something

keep one’s head down - or make sure no one sees you if there is work to be allocated

she’s got a lot on her mind - has many esp. worrying things to think about

have cars on the brain - think about cars constantly

slow-witted - not good at understanding things

to my mind - in my opinion

mind-bending - exceptionally complex and difficult to understand 

keen wits - the ability to think quickly

brain teaser - a problem for which it’s hard to find the answer

not beyond the wit of - not too difficult to do

collect one’s wits (after sth. shocking or unexpected) - make an effort to control your thoughts and feelings

be in one’s right mind - be sane

brainstorm - try to develop ideas

pit one’s wits against sb else - challenge someone over an issue to see who wins

give sb. a piece of one’s mind - tell them what you think of them esp. when you think they’ve behaved badly

MYOB (= mind your own business) - don’t get interested in other people’s affairs

one’s mind is on sth. - one’s thinking about it or giving attention to it

brainwave - a sudden clever idea

brainchild - an idea that one has thought of without any help from others

at one’s wits end - you’ve got no idea what to do next when you’re extremely worried

out of sight out of mind - if you can’t see it its not a problem

sth. takes your mind off a problem  - it stops you from worrying or thinking about your problem

(homework)-minded - thinking about homework (for example)

turn sth. over in one’s mind - think about it continuously

brainless – stupid etc

narrow-minded - unwilling to accept or understand new ideas

use one’s head - think about sth. sensibly

have the wits to do sth. - have the intelligence or understanding to do it

take it into one’s head - decide to do sth. that doesn’t seem sensible to others

should have one’s head examined – behaving as if you are mentally ill

brainwash - make sb. believe sth. that is not true by repeating it again and again

drum sth. into one’s head - keep telling sb. sth. until they cannot forget it

mind-numbing - extremely boring

big-headed - having a very high opinion of oneself

mind your manners - behave yourself

commit to memory - try to remember

exercise one’s brain - exercise one’s intelligence

the words stuck in my mind - I couldn’t forget them

put heads together - discuss a difficult problem among a number of people

the last thing on one’s mind - even if I looked everywhere in my brain I wouldn’t have found that

I can’t get it out of my mind - I can’t stop thinking about it

my mind goes (is) blank - be suddenly unable to remember

grey matter - intelligence

have a good head for - find it easy to learn sth