What makes a song a classic?

Today as I was listening to music I contemplated “What makes a song a classic?” I was listening to “No Diggity” by Blackstreet ft. Dr. Dre and that is a classic hip hop song, so it made me wonder “What makes a song a classic?” There are many songs that we might consider classics. Maybe they became instant classics as soon as we heard them or maybe they became classics over time. For me, I definitely think what makes a song a classic is the lyrics and the beat. What do you think makes a classic song?