Sit tight

It means to wait patiently.

An example of one is :

Our teacher told everyone to sit tight. Don't leave the classroom.

Up in the air

It means things are uncertain or unsure.

An example of it is :

I don't know what our plans are. Things were sort of up in the air the last time we talked.

Piece of cake

It means very easy.

An example of one is :

I thought I was going to fail the test, but it turned out to be a piece of cake.

Twist someone 's arm.

Ow!

It means to convince someone.

An example of one is :

I had to twist her arm a little, but she agreed.

Break a leg

Ouch!

It means good luck.

An example is :

You all look great for the show. Break a leg!