My bicycle pedal fell off today while riding to work. I don't have a wrench at work. How can I install the pedal again so I can safely ride home?
Assuming you normally make effective use of two pedals...
If you have platform pedals, bind your foot to the other pedal using tape or narrow cable-ties. This binding should be weak enough to release your foot if you forget and lean that way, but strong enough that you can turn the crank through 360 degrees using one pedal.
If your local terrain has significant undulations, bear in mind that your power output will be reduced and the leg whose foot is secured will be working more continuously and may fatigue quickly. Consider a more distant tool-source if it offers a flatter route or fewer interruptions.
If you can get it to start on the threads by hand then you can tighten it enough that you should be able to ride carefully home. I've done this before, but its messy (as in, on your fingers).
The threading should make the pedal secure itself (or at least not back out) when pedaling forward, but might get wacky if you tend to backspin or goof off with your feet at intersections. Of course, if your cranks are on backwards...