The English language contains many popular idioms, such as "an arm and a leg," which means something costs a lot; "the ball is in your court," which means that a decision belongs to that person; and the "best of both worlds," which describes wanting all the advantages of a situation. "The last straw" means the final problem in a series of problems.
"A piece of cake" refers to something that is an easy task. When a person wants to express that you should not take a piece of information too seriously, he tells you to "take it with a grain of salt."
"Once in a blue moon" refers to the idea that something rarely happens. When someone's way of doing something seems random and does not make sense to others, he might explain that "there's a method to my madness." To "hit the nail on the head" means to do something in the most effective manner possible.
If someone tells a friend that "every cloud has a silver lining," he means that a negative situation has an upside. When a person says she wants a "penny for your thoughts," it does not mean payment is actually expected; rather, it is a way of asking what someone is thinking.