I wanted the ability to easily open a beer without carrying an extra keychain or card in my wallet. I don't like bulky key chains and don't like carrying extra stuff.

...So here's how I modified the key to my '92 Honda Civic to moonlight as a bottle opener.

TOOLS & MATERIALS:
- Key (car keys probably work better than house keys - so opening a bottle will be a reminder not to drink if you're driving!)
- Bottle with cap still on
- Dremel tool with cutting wheel and high-speed cutter drill bit
- Safety glasses (protection from the metal dust)

Step 1: Figure Out the Geometry of Your Key As It Relates to a Bottle

Making a bottle opener out of my Honda key was pretty straightforward. The groove along the top of the key was just waiting to be modified for the purpose. Hold the key up to the bottle and work out where the key needs to grip under the bottle cap and where you're going to get the leverage from.

I'm sure there are dozens of different key geometries that would work just as well.

Step 2: Cut Away Material With a Dremel Tool (get Under the Cap's Lip)

Based on your key geometry, cut away the material - a little bit at a time - to shape the key to fit the top of the bottle.

I focused first on getting the key under the lip of the bottle cap. I cut a pointed tooth (sharp corners rounded off) that allowed enough of the key head to wrap around to the top of the bottle cap and provide leverage (the next step).

Step 3: Refine the Shape (get the Right Leverage)

To provide the right grip and leverage for the bottle cap, I had to cut out an indent out of the key head that followed the geometry of the bottle cap.

Step 4: Drill a Hole for Your Keyring (if Necessary)

Since I had cut open the groove for the keyring, I had to drill another hole near the edge of the key head. Don't make it too close to the edge so that the key's structural integrity is kept relatively intact.

Also, think about what side you want the keys to hang from (while you're opening a bottle and while you're using the key). I didn't want the keys flopping all over the top of the bottle, so I drilled the hole on the same side as the tooth. The keys hang nicely along the side of the bottle.