Right... I am (rather arrogantly) going to crown myself master of the flame-spitting, and tell you how it should be done. Why? Well, I have some experience in the field, both on how it should be done, and how it should not be done. Recently, I suffered some nice second degree burns doing it wrong, so I might as well share that with you too.
In theory, you can breathe fire with anything that will burn. In practice, there are only very few fluids that are actually usable. You need a fairly particular fluid; It must burn with a low temperature, have a bright flame, must not burn not on any surface. This pretty much means that you have to use some form of paraffin.
To test if you have a good liquid, do the following: Pour some of your liquid in a cup or on the pavement (or on your kitchen floor, if you feel brave). Then try to set it on fire. If it burns, then you do not have the right fluid. Keep searching.
The fluid you want is, as mentioned, a form of kerosene / paraffin / lamp oil. Several firms make fluid that is especially made for breathing fire, which not only tastes minty (a lot better then your regular "Green death" flavour, I tell you that much), but it is also cleaned to the point of being medical-grade. This fluid is made by several firms, and most good pyrotechnics suppliers should be able to help you out. Also, you will not need a pyrotechnics licence to buy it. It is known as "Pyro fluid FS" or similar.
Why a good fluid is important
If you use regular lamp-oil, and you swallow it, you might suffer a nasty medical condition that is known as chemical pneumonia. This is the same as a regular pneumonia, except that it usually attacks both lungs at once, results in about two months in a hospital, and might permanently damage your lungs.
Also, if you get a fluid that will burn on any surface (as in gasoline or alcohol, or whatever), you risk setting things on fire. "things" includes your audience (very bad), yourself (bad) or your surroundings (mildly annoying).
How to practice
Go outside, and bring a glass of VERY COLD WATER with you. Take a sip, and move your head a lot, to get used to have fluid in your mouth. Do NOT swallow any of this water. The reason for using very cold water is that you will notice if you swallow any of it. This is bad (see the point above on why that is important).
Now, spit this out, and take another sip. You will need a small sip of water. Now, open your mouth, and breathe over the water you have in your mouth. Get used to breathing over the water.
Now, spit it out again. Time to get down to business. You know how to whistle, right? Good. Whistle the lowest tone you can. Note how your mouth feels. Now, slowly start whistling in a higher tone, untill you cannot whistle in any higher pitch. Did you notice what you did? Your tongue moved forward towards your lips, making the gap in the front of your mouth smaller.
Right. Now, take a sip of water, and do the same whistling-motion, but with a lot of more force and speed. You should still be breathing over the water, and you will be "feeding" the water to your nozzle (eeh.. lips..) with your tongue, while you breathe the water out in a mist in front of you. The droplets should be as small as possible.
Now for some real business.
Okay, now, get a torch. A juggling torch is ideal (that's what they are made for!), but they are rather pricey, so a regular garden torch or even a tightly rolled up newspaper dipped in your pyro fluid can be a good place to start.
You should first coat your mouth with some full-fat milk. Why? Well, the inside of your mouth can actually take some of the kerosene in, sending it right into your bloodstream. No real problem, as you are unlikely to die from it. However, for the sake of peace of mind, you might as well do it.
Get a wet rag or towel (it should be DRIPPING wet, with water.), light your torch, and put some of the pyro-fluid in your mouth. Preferably a little less than when you were practicing with the water. Now, see if you can hold a torch in one hand, and breathe the fluid approximately 45° upwards. The consistency of water is different from your pyro fluid, so you might have to adjust your technique.
Beware of the wind. To be more specific: There should be none. There are several cool things you can do if it is windy, but because you are a beginner, you shouldn't. So don't.
Okay, now for some fire. Hold your arm straight at 45° upwards, and your torch at approximately 45° upwards as well. Have your wet rag in your other hand. Now, do your trick again, this time into the flame on the end of the torch. You have breathed fire! See how easy that was?
Quickly, after having breathed fire, wipe your face with the wet rag.
More advanced stuff
Okay, don't do anything in this section until you are sure of what you are doing in the section above. Also, make sure someone is standing by with a wet towel and a bucket of water. This stuff IS dangerous.
Try to hold your torch close to your face as you start the flame. When breathing, move the torch with the fluid, while moving your head away from the flame, to make sure it catches on fire properly. Then remove the torch from the flame. You should be able to sustain the flame coming from your mouth for several seconds, without the help from the torch. Looks extremely cool!
Try to breathe fire straight forward. Learn how to control the length of your flame, and you will be able to REALLY scare your audience. However, the key here is control. Most people I have met are not happy if you set them on fire, and if you do, you will probably not be hired for any more pyro shows.
Try to breathe fire downwards. This is dangerous, but it can be done. Make sure you have tried everything above, so you are used to how the flames behave.
Try to breathe fire with somebody, uniting your flames. This is extremely dangerous, and demands that both (or all three, four, five...) of you know very well what you are doing. On the other hand, you can make flames up to 7-8 meters (25 ft) high, which is sure to impress any audience. Make sure you are all standing next to each other (DO NOT FORM A HUDDLE or a circle or anything like that) Make sure the smallest firebreather is in the middle, and the tallest ones are on the side, then all breathe into one torch, held by the middle breather.
Try to breathe two streams of fire across each other (Two people, blowing across from each other). Needless to say, it is a good idea that you both know what you are doing.
Try to light one fireball with another one. Have a row of firebreathers stand next to each other. One lights their fire from a torch, the next from his (or her) flame, etc. Looks cool.
Try to breathe fire laying down (note that you cannot move your head backwards, and that this is probably one of the most dangerous things you can do when breathing fire on your own).
Never, ever, breathe fire if you cannot breathe through your nose because of a cold or whatever. (I have done this, and it resulted in me gasping for air when I had pyro fluid in my mouth. I swallowed a lot of liquid, and was sick for two days)
Never, ever, breathe fire in strong winds (I have seen someone doing this, and he almost set his house on fire)
Never, ever, breathe fire with the wrong liquid (I did this at a nodermeet, and set my face on fire, resulting in second degree burns. Painful for a week, and worse: rather embarassing)
Never, ever, use petrol, alcohol in any form or gas (as in lighter gas, protane, butane or whatever) to breathe fire. In order, using those substances are dangerous (will set your face on fire, and is poisonous), useless (burns with an invisible flame, and will set your face on fire), dangerous (you might suffocate, as you will definitely not be able to control gas, besides, there is a danger of explosive fire backfiring into your mouth)
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