For a class, we made a hot air balloon. It is very simple, if done correctly. Regardless of how you make it, the same essential materials needed will not change.
We got our ideas from another instructable, similarly titled DIY Hot Air Balloons by TheScienceMan. I would recommend viewing that if you want to do it right. Or at least differently. Keep in mind that ours didn't work for whatever reason.
6-8 small birthday candles
Plastic bag-thin-can be dry cleaning bag
Four six-inch wires-can be baling wire
Card stock for the base(four-inch square base)
A few strips of Duct tape (not strictly necessary)
Hot glue gun-for attaching the candles to the base
Lighter (for the actual flight)
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Step 1: Cut Out the Base and Wire
First, you should cut out a four inch square in the card stock, and cut the wire into four six-inch strips. If you want, you can also wrap the card stock in a thin layer of duct tape to prevent it from having a chance of burning.
Step 2: Attach Candles
Once you have the base, you can gather the six to eight candles and either hot-glue them directly to the base or wrap the lower ends in the tape and then do so. The candles won't melt onto the base if you use the tape, but them melting into the base would work just as well.
Step 3: Attach Wires
Now that you have the base and the candles, you can use the scissors (or anything, really) to poke small holes at each corner of the base, just big enough for the wires. Then you put the wires through, bending the last inch on the underside of the base so that they are loosely held upward.
Step 4: Prepare and Attach the Bag
Before the bag can be attached to the wires, you need to tie four equidistant knots in the opening. Doing so will greatly reduce the size of the opening, so that it shouldn't be much larger than the base. Then you can attach it. Simply hook the unbent ends of the wires through the knots, one to each, and voila. The hot air balloon is built.
Step 5: Ignition
Now that you have the hot air balloon, go to an open area, or at least one that you are least likely to burn to the ground if something goes horribly wrong. There, ignite the handles, being sure not to melt the bag or incinerate the base. Once they are lit, you can pull the wires down so that the bag is fully covering the base without any gap and fold them off to keep it from moving again. The bag should fill with hot air and begin rising, if you did everything correctly. Or at least more so than us.