Picture this, you're riding over to the park to meet your friends for a BBQ (winter BBQs are one of the luxuries of living in California or the southern hemisphere) and you get a flat tire :(.
No problem. You've got everything you need to fix a flat in your back pack... except you took your pump out of your bag to make room for all those burgers, buns, and beer you're haulin'. Uh-oh.
There's a gas station on the corner, but their air pump is for schrader valves (like on cars, laaaaame) so you're SOL, right?
Wrong! You've got a makeshift presta to schrader adapter right on your tube! Let's do some magic.
First, remove your valve cap and thank your lucky stars that you read this blog post before you left the house.
Now, you're going to cut the tip of the cap to make a nice adapter-y shaped tube!
Ooooooo so tubey.
Now flip it over and put it back on your valve (don't forget to open the presta valve first by loosening the tip).
Boom, instant adapter! Shove some air in there with that schrader pump you've got.
And you'll be good to go!
You can even screw your newly made adapter back on the valve when you're done and you'll be ready for the next time (and it's less likely to be stolen than if you leave a real presta → schrader adapter on there)!
Though it might cost you some style points.