From FAR Part 1.


Flight time means:

(1) Pilot time that commences when an aircraft moves under its own power for the purpose of flight and ends when the aircraft comes to rest after landing.

Ok so does this mean you can do a 15 min taxi and 15 min run up and do one touch and go in the pattern and taxi back for another 15 min and legally log 1 hour in your logbook? I know this example is extreme and not the norm.