1. When pigs fly - an event which is highly impossible to happen.
I'll pass that test when pigs fly.
Meaning: It is highly impossible to pass that test.
2. Baker's Dozen - thirteen.
Can I have a baker's dozen of cookies?
Meaning: The person is asking for thirteen pieces of cookies.
3. Dead Ringer - identical.
I'm very sure that this is a dead ringer of my work.
Meaning: The person is sure that his work is identical to another person's work.
4. Pig out - to eat a lot and to eat very quickly.
You sure do pig out very often.
Meaning That person eats a lot and very quickly often.