“Piece of cake,” “costs an arm and a leg,” “hit the books,” “let the cat out of the bag” and “you can’t judge a book by its cover” are some common idioms and phrases. An idiom is a phrase that has different cultural meaning as opposed to the literal meaning of its composite words. “Ace in the hole,” “Achilles’ heel,” “at the drop of a hat,” “bite the bullet” and “kick the bucket” are also idioms.
“Piece of cake” is used to indicate that something was or is very easy to accomplish. “Installing the new software on the computer was a piece of cake” is an example of it in use. “Costs an arm and a leg” means that something is very expensive; for example, “The newly released motorcycle costs an arm and a leg.”
“Let the cat out of the bag” means to reveal a secret. “You can’t judge a book by its cover” means that one cannot judge or understand a thing based on its external appearances. “Ace in the hole” means a secret strength or advantage that is not revealed. “Achilles’ heel” is used to indicate a critical weakness in spite of extraordinary strength. “At the drop of a hat” means to do something instantly or without any hesitation.