Idiom: A piece of cake
Meaning:  This idiom is used to describe a task, job or other activity that can be accomplished very easily, just like serving a slice of cake, very simple and pleasant.

Examples: 

• My homework is a piece of cake; I can finish it within an hour.
  
• Riding a bike is a piece of cake.
  

Idiom: It's raining cats and dogs
Meaning: It's raining very heavily

We use this idiom to say that it is raining VERY heavily. It does not actually mean that cats and dogs are falling out the sky!

Examples:

• The weather was horrible on Saturday. It was raining cats and dogs all day.
• It's really awful weather outside. It's windy and is raining cats and dogs.

Idiom: Be the apple of someone's eye
Meaning:  Be loved very much by someone; someone's favorite person or thing; a boyfriend or a girlfriend.

If someone is the apple of your eye, you think he/she is very important to you, and you love him/her very much.

This idiom is used especially when someone is loved by an older member of his/her family. 

Examples: 

• Kelly is Bob's only granddaughter, and she's the apple of his eye.
  
• Sarah's only son was the apple of her eye.

Idiom: Grab a bite (to eat)
Meaning: Get and eat food quickly

If you grab a bite (to eat), you get food and eat quickly because you are busy or in a rush.

Examples:

• We have five minutes before the show. Let's grab a bite.
  
• Tim often grabs a bite to eat between meetings.
• Last night, we grabbed a bite before we went to the movie.
• Wanna grab a bite after work?