Idioms are phrases that carry a different meaning than the words that form them.
Please share your examples of idioms here:

Piece of cake - Something that is very easy.

a. If you know the material, the test will be a piece of cake.

b. Can you show me how to use this program? Sure, piece of cake.

On the house - free, paid for by the establishment.

a. Because the restaraunt is celebrating its anniversary, all drinks are on the house.

( to be paid) Under the table - discretely and without paperwork.

a. Because Maria is paid under the table, she does not pay taxes on her earnings.

Tie the knot - To get married.

a. After three years living together, Max and Anita finally decided to tie the knot.

Barking up the wrong tree -

A chip off the old block -

On cloud 9 - extemely happy
a. Maxwell was on cloud 9 after Maria kissed him for the first time.

Out of one's head

Play it by ear

Set in stone

Nailed down

Sooner or later

Fly by the seat of your pants

Make it happen

Laid off, lay off, layoffs

The ends justifies the means

Think on your feet - To think or respond quickly.
a. A good manager has the ability to think on his feet and deal with unexpected situations.

Dragging his feet

Read my lips

It will pay for itself

Flea market

Class clown

Lose your temper/cool

A no brainer

Play devil’s advocate

Set the bar very/too high

The lion’s share

Red tape

Cold feet

Set (sth.) in motion

Look (sth.) up in the dictionary (see phrasal verbs)

Kick out/off

Release a statement

Shut (so.) up (see phrasal verbs) -

Take them to the cleaners -

Par for the course -

Plan on (doing something) -

Head (his/her/my) way -

Bully pulpit -

To take (sth.) with a grain of salt -