Remember Back to the Future?
How, by changing one small event in the past, Marty McFly endangered his own existence?
Now, think about that teacher in your life. The one whose impact made you who you are today. What would the alternate reality be if that teacher were never in your life? Would you still exist as you are?
It may have been a teacher in school or in lessons. Or, it may have been a friend, parent, or mentor who did the teaching.
We all have at least one teacher in our lives who made us who we are today. If they're still around, why not send them a note to say thank you?
And in turn, who can you teach today?
You never know which experience will cause that ripple effect that affects the entire space-time continuum.