If you’re biking along and need to tighten something up on your bike, like your bike seat, here’s a trick to remember even if you forgot your hex wrench.
This Instructable says that as long as you have a knife and pencil, you can carve an emergency wrench.
Shave the sides of the pencil down to size. Keep the peelings parallel to the faces. Make your shavings pretty thin. Test to see if it fits….Insert your new allen wrench into the bolt head. Turn slowly. If the wrench slips, it has been stripped (wood is soft), [recarve]
Riding with loose screws is dangerous, so you’ll want to get to a real wrench as soon as possible. To be totally safe, though, you should walk your bike until that time.
How to Make an Emergency Hex Key / Allen Wrench [Instructables]