common knowledge

Common knowledge is information that is well-known to everyone.
“The intern is Jack’s son – I thought that was common knowledge!”

get wise to something

If you get wise to something, you learn something that you were not aware of before.
“The old man finally got wise to the fact that children were stealing apples from his garden.”

know better than to do something

If you know better than to do something, you are experienced or wise enough not to do it.
“You should know better than to go sailing in stormy weather.”

know which side your bread is buttered

If you know where your interests lie or what will be to your advantage, you know which side your bread is buttered.
“Paul never argues with his father-in-law because he knows which side his bread is buttered.”

know which way the wind in blowing

If you know which way the wind is blowing you know how a situation is developing in order to prepare for any changes.
“Before we decide, we need to know which way the wind blowing.”

know the ropes

Someone who knows the ropes is familiar with the way something is done or knows how to do it.
“Charlie can fill in for Sam. He knows the ropes.”

know the score

If you know the score you are well-informed about a situation and know what to expect.
“If Laura damages the car, her dad won’t lend it to her again. She knows the score.”

know someone inside out

If you know someone inside out, you know them very well.
“Sue and Anne have been friends since childhood. They know each other inside out.”

know something like the back of your hand

If you know something like the back of your hand, you are very familiar with it or know it in detail.
“Of course I won’t get lost. I know London like the back of my hand!”

know your own mind

If you know your own mind, you know what you want or like, and are capable of making a decision.
“I don’t want to influence you. You’re old enough to know your own mind.”

knowledge is power

This expression means that the more knowledge you have, the better your chances are of being successful, or the more control you will have over your future.

street smart/street wise

A person who is street-smart or streetwise has enough experience and knowledge about life in the city to be able to deal with difficult or dangerous situations.
“The kids living in this area are all street-smart – they’re in less danger than us.”

tech savvy

People who are tech savvy have sufficient technical knowledge and skills to be comfortable using computers and other electronic devices.
“Many students are more tech-savvy than their teachers.”

(a) walking encyclopaedia

This term refers to a person who is very knowledgeable about a lot of subjects.
“The origin of Halloween? Ask Jill – she’s a walking encyclopaedia!”

wise for their years / wise beyond their years

Someone who is wise for their years or wise beyond their years has more knowledge and experience that most people at their age.
“She’s still a child but she’s wise beyond her years.”