What is an idiom? It's a combination of words that form a natural expression.
"A penny for your thoughts?" - We ask someone this when it looks like they are deep in thought. This usually coaxes them into talking about whatever is on their mind.
At the drop of a hat - If someone does something at the drop of a hat, it means spontaneously and without any prior planning. Ex: Mark starts gambling at the drop of a hat.
Beat around the bush - To not address the issue directly. Ex: Politicians are always beating around the bush when it comes to improving the healthcare system.
Costs an arm and a leg - It's very expensive. Ex: The new iPhone X costs an arm and a leg.
Don't put all your eggs in one basket - Don't risk everything in one area. Ex: Don't put all your eggs in one basket. Invest in many different stocks instead of just one.
Kill two birds with one stone - Accomplish two tasks with one action. Ex: You can kill two birds with one stone if you do homestay in Canada. You'll get a nice vacation and be able to practice your English.
Not playing with a full deck - The person is a little crazy. Ex: My neighbor throws beer bottles at birds from his balcony. He isn't playing with a full deck.
Once in a blue moon - Happens rarely. Ex: I see my aunt once in a blue moon because she lives so far away.
Wouldn't be caught dead - Would never do X. Ex: Tom wouldn't be caught dead wearing a green suit.