Uptown teens

There I was walking behind a group of teenagers in an upmarket area of Warsaw. A smart and shapely 16(?)year-old female challenged her three male companions:

"Guys! Why do sperms have tails?!"

The boys jumped around her offering a few of their own ideas, to which she replied with a complete assurance:

"So you can pull them out of your teeth easily".

Laughs all round.