Thank you

Thank you is a common phrase used to show you are thankful of someone. It is a very old phrase, coming from as early as the 1400s. "Thank you" is often replied with "you're welcome". 
People often say "please" to request something to you. If they are satisfied, they will say "thank you" back to you.
The phrase "thank you" can be modified to have a different meaning. For example, "no, thank you" or "no thanks" are often used to politely say no to an offer. Common responses for "thank you" include "you're welcome", "don't mention it", or, more recently, "no problem". Sometimes, the phrase "thank you" is modified to show that a person is not thankful of someone, such as the phrase "thanks for nothing".