Greetings, fellow makers and craftsmen (and women!)

Is your house occasionally visited by unwelcome rodents? Do you organize underground mouse fights? Or perhaps you've had enough of burned ratatouille?
If you've answered 'Yes' to any of the above, then rejoice, for the instructions herein may be of some help.

The foot-long BMT - that's Bottle Mouse Trap, of course - can be used anywhere from subways to budding Michelin-starred restaurants.

Say cheese!

Step 1: Supplies

You will need the following materials

Empty 2-litre plastic bottle with smooth sides (see picture). This will serve as the main body of the trap. The reason for smooth sides is that the captured rodent will have a hard time pushing its way out.
A second plastic bottle similar to the first. We will only need the middle section (the smooth sides) of this bottle. This will be used to reinforce (stiffen) the sides of the first bottle.
PVC pipe. 20 to 30 mm diameter should do nicely. The one I'm using here is 45 cm long.
Metal wire - 30 cm should be more than enough. The piece shown here is 1.4 mm in diameter, but you could use anything that's able to withstand sustained gnawing from a desperate rodent.
Elastic strip, the wide type used in clothing. The required length will depend on several factors: elasticity, bottle size, length of PVC pipe, and length of wire. Trial and error is your friend. Remember, it is better to err on the long side, since you could always halve it by tying the two ends together, forming a loop (although this will result in a stiffer elastic band)
Short piece of dowel / stick / pencil - this is simply used to secure the end of the elastic band to the end of the PVC pipe. Obviously, there are numerous other ways to do this.

Tools

Blade
Drill
Pliers
PVC cutter / hacksaw

Safety

Gloves - for hand protection while handling the sharp blade and drilling.

Step 2: Operation

This is a spring loaded trap. The main body chiefly consists of the bottle bottom and PVC pipe, which houses the strong elastic band. The trap door consists of the bottle top, which rests against the bottom edge of the bottle (at the six o'clock mark). Also, part of the trap door touches the ground. This is important!

The trap is triggered whenever something pushes down on the rim of the main body. This will cause the cap to slip upwards, slamming the trap door shut, and conveniently pushing the triggering agent (i.e. the rat) in the trap.

The trap has to be manually reset each time it is triggered.

The picture annotations cover everything else you need to know.

Step 3: Construction

The construction steps are quite straightforward.

Cut bottle at the point where the walls of the top part straighten. See picture.
Drill small hole through bottom of bottle. The hole has to be big enough for the wire to pass through.
Cut wire. It must be 5-10 cm longer than the bottle bottom.
Drill hole through the cap. It should be slightly off centre (more on that later), and the same size as in step 2. It should also be as neat as possible, as it is the single point at which the trap door is pulled by the wire and should sustain the repetitive impact of the trapdoor slamming shut.
Use pliers to form the end of the wire into a relatively bulky stopper 'knot'. It must not be allowed to slip through the cap!
Thread the wire through the cap, then through the bottom of the bottle.
Now make a loop with the other end of the wire, and attach it to your elastic strip / loop.
Drill hole in one end of the pvc pipe. The other end will be in contact with the bottom of the bottle.
Using another wire, thread the elastic loop through the pvc pipe. Insert stick through the holes drilled in the previous step, making sure it also passes through the elastic loop.
Time to adjust the the elastic loop's tension. Pull the trap door open and feel the tension. It should be strong enough to slam the door shut in the blink of an eye when released. Rodents are quick, and will approach your trap with caution, ready to flee at the slightest danger. The faster the trap door closes, the better.
The elastic band should still be reasonably tensioned when the trap door is closed. A captured animal will try to push the door open, and will throw everything it's got at it.
(optional) Once you're happy with the tension in the elastic band, remove the wire that was used to thread it through the pipe.
We will now reinforce the sides of the bottle. This is necessary to prevent the bottle sides from buckling when the trap is armed. I have chosen to sew the two layers of plastic together. Alternatively, you could use duct tape, or even a staple gun. Remember, the trap is likely to remain in the armed position for quite some time, and the whole structure should be rigid enough to stay together under continuous tension by the elastic band.
If you're sewing, then you will need to drill / pierce small holes along your seam line. Even with rudimentary skills with a needle, this should not take more than a few minutes.
Once the reinforcement is complete, the next step is to fine tune the trigger mechanism.

Step 4: Setup

Now that the device is complete, you need to:

adjust the trigger sensitivity according to your needs - see paragraph below.
choose an appropriate bait. In my particular case, I used a whole passion fruit.
choose an appropriate location.

Sensitivity

Since the cap was drilled slightly off centre, you can adjust the trigger sensitivity by rotating the trap door clockwise or anticlockwise. Although to a lesser degree, you can also tweak the sensitivity by moving the point where the cap touches the bottle rim a few millimetres left or right. It all depends on your craftsmanship, really, and trial and error is the way to go

Additionally, you want to minimize two situations:

The first, false positives, occur when the trigger is too sensitive (triggering without any animal).
The second, false negatives, occur when the trigger is not sensitive enough (failing to trigger despite entry of critter).

Increasing the trigger sensitivity means you can capture smaller, lighter critters, but this will inexorably increase the chances of getting false positives. Conversely, decreasing the sensitivity will lessen the chances of the trap going off by external factors (wind, rain, a leaf falling onto the trap, an inquisitive dog..), but doing so will enable smaller, lighter critters to get in the trap and walk right back.

It therefore boils down to this: Decreasing the chances of getting false positives will automatically increase the chances of getting false negatives, and vice versa. You must therefore adjust your trap to your particular situation. How big do you think your target rodent is? Where will the trap be located? What are the chances of getting false positives? Will your pet dog have access to the armed trap? Will your bait attract non-targets?

Bear in mind that there are many environmental variables at play here, both expected and unexpected. You may not know them all, but that's fine. Set it up and adjust course along the way.

Baiting
Choose your bait according to your situation.
If you're reading this instructable, chances are you have an unwanted rodent in the house.
What are its eating habits? Does it have a particular preference for certain foods? Try those first.
Note that most baits will potentially attract other hungry creatures. Depending on your location and choice of bait, you can expect to see ants, cockroaches, snails, squirrels, moles, etc..

Location
Where should the trap be placed? Indoors or outdoors?
Again, this will depend on the particular habits of the offending rodent. As a general rule, try to keep it sheltered from the elements, and out of reach of non-targets (pets and children, for instance)

Step 5: The Hunt

You are now ready to rock.

Pull the trap door open.
Latch the cap onto the bottom of the rim of the bottle (at the six o'clock mark).
Ensure the trap door also touches the ground (see picture).
Sit back and relax. Go about your day.
Check on the trap from time to time. Check the bait.
If you have false positives, your trigger may be too sensitive. Adjust accordingly.
Eventually, your patience will be rewarded!

Step 6: Results

Now what?

To kill or not to kill, that is the question - the moral, philosophical, religious and spiritual implications of which are beyond the scope of this instructable. The reader be the judge.

How to kill a rat (or any other creature, for that matter)

Freeze to death
Set on fire
Drowning
Electrocution
Asphyxiation
Poison
Moving target shooting
Use as live bait
Let your cat take care of it

A small selection of potential uses for a dead rodent

Dissect it and learn about rodent anatomy
Clean its bones and mount the skeleton for display
Cook and eat it. Wild rats taste like pork, they say.
Feed to your pet owl
Have a go at taxidermy
Make fur coat for your hamster
Make potions
Make a real dead rat prank
Throw in the compost bin!

If that last point seems strange to you, consider the following excerpt from Joseph Jenkins' fascinating book, The Humanure Handbook, 3rd ed. (pp 62-3). The book can be read, free of charge and in its entirety here.

Composting is considered a simple, economic, environmentally sound and effective method of managing animal mortalities. Carcasses are buried in a compost pile. The composting process ranges from several days for small birds to six or more months for mature cattle. ... Animal mortality recycling can be accomplished without odors, flies or scavenging birds or animals.

Should you opt not to kill, you may consider the following options:

Employ it as head chef in your Michelin starred restaurant
Train it to sniff out land mines
Adopt as wild pet
Offer to laboratory for research?
Use it in your school science project?
Prank your beloved friends?
Last but not least... Set it free!

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EDIT:
If you have successfully caught a rodent, and are planning to reuse the same trap, one important point to bear in mind is that the trap will carry the smell of the previously caught rat. Unless you give it a good clean, it is likely that other rats will avoid the trap.

Introduction: Recycled Bottle Live Mouse Trap