Unless your bike is total crap, chances are you've got a pair of Presta valves to deal with each time you pump the tires. All's good and well if you're carrying the right pump, of course. But what if you get caught without and have only a gas station air pump to resort to?
Air pumps at gas stations have hose nozzles made for the Schrader tire valve found on virtually all cars and motorcycles, as well as most bikes. And as with all French things when it comes to cycling – the mechanism behind the Presta valve was invented by a certain Monsieur Sclaverland at the end of the 19th century – French and standard parts don't get along.
In this case, it's a matter of different valve stem designs. The standard Schrader stem is 8 mm (5/16") thick and uses a recessed, spring-loaded pin to keep the air in; the Presta valve stem is thinner, at 6 mm (1/4"), and has a threaded pin that sticks out, with a knurled nut which you must unscrew all the way in order to pump air in. This mechanism allows you to pump Presta tubes to the much higher tire pressures used with "skinny" tires seen on road-bikes and fixies. Just like the Apple Lightning connector, the thinner Presta valve stem allows for narrower, lighter rims. (The picture to the left shows a Presta in the closed position; and the lead image above shows a close-up of it.)
Presta valves also come with a little cap to prevent this exposed pin from getting bent and to stop it from puncturing the tube while in its packaging. Bike tinkering demi-god Sheldon Brown thought little of this cap, proclaiming that "there's no reason to use them in actual riding." WRONG. It's actually this part that you can literally hack to bridge the two valves' design differences. You just need a blade.
See that rounded smooth top part? Hack it off with your blade. Now open your Presta valve, thread on the cap, and grab the compressor nozzle. The extra width the cap adds to the valve stem acts as a pipe, channelling compressed air into the tube faster than it can escape. You need compressed air for this to work — a common floor pump won't; any air you pump in will escape during your upstroke. And while most gas station air pumps top out at 60 psi (typical "skinny" tires need around 100 psi) it'll give you enough tire pressure to prevent your bike wheel from "bottoming out" when you go through a pothole, which could result in a flat or even mangle your fancy rims.
Presto — you can now pump your Presta at the gas station!
[top image via Flickr user pws5252; Presta valve stem image via Wikipedia.]