This is probably one of the most useful things ever invented. Air tanks have more uses than i can explain. This is a fun project in which you can create a low pressure pvc air tank in minutes. It will require very little effort and the air tank consists of only 5-6 parts which will cost you less than 4$ If you get your supplies from the right places. So lets get started.

Step 1: Supplies:

There really isn't much to it. All you need is...
- pvc pipe(any size will work, i used 3/4"x18" of pipe but the bigger the better:)
-matching pvc end caps( in my case i would be using 3/4" caps)
- bike tire valve, also known as a Schrader valve
- garden hose nozzle
- threaded soda bottle spout(Optional)

Tools:
- hot glue, super glue, or plumbers glue
- drill or knife

Step 2: Begin

Ok so one thing you should know before you begin is that the threaded soda bottle spout is only nessasary if it can screw directly into the bottom of you garden hose nozzle. Otherwise you will not need it. Luckily, the nozzle i got from the dollar store fits perfectly on most soda bottles, so i will be using it.
To begin, you can go ahead and drill some holes in your pvc end caps,using either the drill or a knife. one hole in one cap should be a bit bigger than the other, but the smallest hole should be just big enough to fit the bike tire valve through it, which is about 1/4" in width.

Step 3: Schrader Valve

The bike tire valve(or Schrader valve) should be glued to the inside of the hole you just drilled, make sure it is air tight with no air leaks. Schrader valves can be found connected to a bike or car tire, but you can get them for free at most bike shops. Bike shops usually keep a box full of leaking bike tires behind the till, usually the people working there will give you as many as you want for free, you can then just cut the valves out of the tires and use them. Now that your done with the first pvc cap you can move onto the one with the bigger drilled hole. This hole should be anywhere from 1/4" to 1" in width.

Step 4: The Other Cap

So this step is skippable if you don't happen to need the threaded soda bottle spout, but for those who do here is what must be done...
Very simple, just glue the spout directly on top of the larger hole you drilled, again make sure this is air tight by screwing on the garden hose nozzle tight and blowing into the cap, listen for air leaks. To get the threaded soda bottle spout, find a soda bottle which screws into your hose nozzle perfectly, then saw off the end.
If a soda bottle spout is not at hand, instead of doing this step just glue the hose nozzle ontop of the drilled hole.

Step 5: Glue It All Together

Now to make the body of the tank, take your pvc pipe and glue the pvc end caps to each end. note: i am only using hot glue because i will be using this tank for low pressure uses only, but for high pressure your going to want to use super or plumbers glue. *Make sure all pvc is pressure rated*.

Step 6: Finishing Up

There isn't much more to do from here, just screw on the nozzle and your ready to go...
*how to use*
The air tank can be filled using either a bike pump or an air compressor. To fill, just connect the hose from either your pump or compressor to the Schrader valve and fill to the needed pressure. Then to release the air press the trigger on the hose nozzle and air will stream out of the tank. This can be used for a number of things, including being the power source of your totally awesome air soft gun!
REMEMBER: I am not responsible for the ridiculously dangerous things which are made possible by this device, make at your own risk!:p