fixing a flat

on monday i got a flat on my rear tube (in the rain). boohoo. today i changed out the tube and felt along the inner surface of the tire and found no sharp debris. so the cause of the flat is unknown. i even inspected the old tube and it looked pretty ok to me. but maybe the lighting is poor in the garage i don't know.... anyway, as an exercise i timed myself to see how long it would currently take me to change a flat, and all told, including frustrating and extraneous "fitting the wheel back into the dropouts" time, it was 27 minutes. not bad, but if that happened on my way to work i would pretty much be fucked. and i have one stray scratch from the derailleur cable. let this be a record to be compared against my next flat-changing experience.