Nature
Lightning fast.
Extremely fast
It's raining cats and dogs.
It's raining heavily.
A piece of cake
Very easy
To pass with flying colours
To pass something easily/with a high score
To feel under the weather.
To feel ill.
Snug as a bug
Very comfy
A couch potato
Someone lazy
To get the green light
To get permission to do something
Once in a blue moon.
Happens very rarely
Quiet as a mouse
Extremely quiet
To have bigger fish to fry.
To have more important things to do.
To see red
To be extremely angry
On cloud nine.
Very happy/joyous
Eating like a horse
Eating a lot
Two peas in a pod
Two things that are (nearly) identical/extremely similar
Caught red-handed
Get caught doing something wrong
Every cloud has a silver lining.
Every bad event has a positive aspect
Kill two birds with one stone.
Solve two problems with one action.
Eating like a horse.
Eating a lot.
Green with envy
Extremely jealous