There's one language used in every country. People who use it are young and old, short and tall, thin and fat. It's everybody's second language. It's easy to understand, though you can't hear it. Do you know what it is?

有一种语言在每个国家都使用，使用这种语言的人有老也有小，有高有矮，有胖有瘦。这种语言是每个人的第二语言。虽然我们听不到它，但是这种语言很容易理解。你知道这种语言是什么吗？