Who uses it: Physicists and science-fiction writers
What it means: An argument against time travel -- if you could travel back in time and kill your own grandfather before he met your grandmother, you would never exist, so you would not be able to kill your grandfather.
How you can use it: To make yourself dizzy.
I'm fond of time travel stories -- Jack Finney's Time after Time and Connie Willis' To Say Nothing of the Dog are my favorites, and I admit that I love Diana Gabaldon's Outlander, too.
Kevin Wignall, on his Contemporary Nomad blog, recently posted the advice his time-traveling self might give to his younger incarnations. My advice to my younger selves probably wouldn't be that different from the advice I'd give myself right now: 1) you are not as central to the process as you'd like to believe; 2) everything manages to work itself out, the vast majority of the time; 3) don't let yourself get too isolated; 4) ask for help, dammit.
If I could travel back in time, I wouldn't bother with anyone as boring as my younger self. I'd pillage with Vikings, or sit in an audience at an Oscar Wilde lecture, or consult the oracle at Delphi (although, if I were coming from the future, I'd already know the future, so what would be the point of that?). Where would you go, in the Wayback Machine?
It snowed at least four inches overnight, and the snowplow man came to my parking lot at 5:30 a.m. When he comes during daytime or evening hours, he honks for the residents of this building to move our cars; wisely, he did not honk at 5:30 this morning.