Walk into a bar. Make direct eye contact with the bartender, but say nothing. Leave the bar so that the bartender cannot see your exit, either through a back door or bathroom window. Make note of the exact date and time at which you entered the bar, and also of what you are wearing. For the next twenty years, travel back in time to that same day and enter the bar one minute later than you entered the year before, wearing the same outfit. If properly executed, the bartender will see twenty versions of you, each slightly older than the last, walk past and nod him hello, over the course of twenty minutes. For added effect, come back to the bar 21 minutes after your initial visit and order a beer. Act like nothing has happened, and staunchly deny that you have been in the bar before. If your future self was committed enough, the bartender’s mind will be blown.
Creating A Paradox
Perform the Deja-Vu prank as outlined above. If the bartender’s mind is blown when you reenter the bar at the 21st minute, indicating a dutiful and time-travelling future you, destroy your time travel device right then and there, thus rendering full completion of the prank impossible. Sit back and relax as spacetime itself is rent asunder by the power of your intentions.