When a male ties his penis to a string and ties the other end to a spring block system where the spring is attached to the wall and pulls the block back and releases thus executing simple harmonic motion described by the classic differential equation.
Song selling takes place when a singer-songwriter earns money in exchange for the rights to use his or her original music, often to a (more) famous, or up-and-coming, recording artist.
If a singer-songwriter has written a catchy song but seems to fall short on vocal or instrumental ability, a listener might say, "you should sell your song." Hearing this can be extremely disheartening for singer-songwriters, especially those who are still developing their talents, or have not yet enjoyed their fifteen minutes of fame.
Why does song selling even exist? Any band or singer who is worth their weight can write their own songs.