This is another tweet playing with the “opening line of my life story” tweet meme. In this case, we are describing a line using the function “sine.” That’s the wavy line that goes up and down around the horizontal axis. It starts at zero, goes up to one, goes back down through zero, all the way to negative one, and back up to zero. Then it repeats.
So that’s most of the joke. If you believe in reincarnation, then your life line would repeat. Like a sine wave.
While the sine wave starts at zero and goes up, there is a similar wave called the cosine wave, that starts at one and goes down. If you saw the movie, “The Curious Case of Benjamin Button” then you know the idea is that the main character starts old and gets younger. The cosine starts high and goes down, hence it is like old Ben getting younger as time goes on.
Homework: Go make a tweet about the homophones cosine and cosign. People will think you’re hilarious. (They won’t. They really won’t.)