Complex sentence: Sticking one's tongue out at someone is considered a childish gesture of rudeness and/or defiance in many countries; the act may also have sexual connotations, depending on the way in which it is done.
Split into simpler sentences: Sticking one's tongue out at someone is considered a childish gesture of rudeness and/or defiance in many countries. The act may also have sexual connotations, depending on the way in which it is done.