Animals
It's raining cats and dogs.
It's raining very heavily.
Break a leg.
Good luck.
Saving for a rainy day.
To save something for later.
Piece of cake.
Easy.
You can't judge a book by its cover.
Don't judge something/someone by how they look.
Kill two birds with one stone.
Accomplish/do more than one thing with the same action.
Costs an arm and a leg.
Very expensive.
Take a rain check.
To postpone plans.
You can't have your cake and eat it too.
You can't have everything.
Burn bridges.
Destroy relationships.
Barking up the wrong tree.
To be mistaken, or to be looking for solutions in the wrong place.
Give someone the cold shoulder.
Ignore someone.
Rain on someone's parade.
To spoil/ruin something.
An apple a day keeps the doctor away.
Apples are healthy.
Fit as a fiddle.
To be in good health.
Birds of a feather flock together.
People who are alike are often friends, (meant in a negative way).
Wrap your head around something.
To try to understand a difficult concept.
A storm in a teacup.
A big fuss about a small problem.
You can't make an omelet without breaking a few eggs.
There's always a cost to doing things.
Out of the frying pan and into the fire.
Things are going from bad to worse.
A bird in the hand is worth two in the bush.
What you have now is more valuable than what you may have later.
By the skin of their teeth.
Just barely.
Once in a blue moon.
Something that rarely occurs.
Don't cry over spilt milk.
There's no reason to complain about something that can't be fixed.
They're not playing with a full deck.
To be dumb, calling someone dumb.