"X" stands for the unknown in mathematics. Replacing one's name with an unknown number adds infinite laughter and romantic mystery to everyone's guess. In addition to the unknown, "X" has another meaning: kiss. This is mainly because the pronunciation of "X" is similar to that of "Kiss". At the end of letters, emails or cards, many people will write: XOXO. If "X" is "kiss", then "O" is "hug". Don't we always put our arms in an "O" shape when we hug?