Alright this one is easy there are 3 methods to getting a fireball. Just so you know all cans must be flammable.
Method #1: All you need is an aerosol can, some napalm i.e. (Styrofoam and gasoline or acetone dissolved into a putty) or other flammable putty. A lighting device i.e. matches, lighters.
Spread napalm all over the can and light with matches, and in about 30-60 minutes the can will rupture spraying flammable liquid into the air as it gets caught on fire. Make sure you stand far back from this. This method is by far the longest and very boring.
Method #2: You need a can, lighter or matches, a bb gun pump or co2. for this one you will need two people.
Spray a flammable liquid onto the can and light. Then have someone shoot the can and then as soon as the bb punctures it there will be a fireball. This one is easy and fun. If you have a pump bb gun pump it at least 10 times.
Mehtod #3: This is the most entertaining method for this one you need to exercise extreme caution and good judgment. That being said you need a COB aka Crater Maker. A can, common sense. Ok take a cob and wrap tape around the aerosol can. Make sure you have at least 6 inches of green visco fuse on that cob. Now light that fuse and RUN. There will be a loud explosion and then a nice fireball to boot. Be careful with this metal shrapnel is deadly it can kill. Some people mentioned about how there cobs didn't puncture the can well in that case you might want to consider using some bb's, for this all you do is attach a few bb's to the inside of the can. So now your bb's are sandwiched between the cob and can.
In my experiences the best aerosol cans to buy are: starter fluid, hair spray, flat tire sealant(fireball raining down flammable foam), spray paint cans. All in all spray paint is the best because it's like a dollar a can and it's easily available.