Materials:
Balloon
Gas Can with nozzle narrow enough for the balloon to fit over (not completely full, but at least 1/4 full)
Post-It or paper and tape
Step 1: shake the gas around in the can lightly.
Step 2: extend the nozzle and put the balloon over the nozzle
Step 3: without inhaling any gas or vapors, blow into the other nozzle that lets air in when gas is pouring.
Step 4: Once you have blown enough gas vapor into the balloon so that it is full, take off the balloon and tie it off.
Step 5: Put the post it on the balloon or tape the paper to the balloon so that there are a couple inches left hanging away from the balloon.
Step 6: Light a corner of the paper and back up about 10 feet.
As soon as the fire melts the balloon, the gas vapors will escape, then ignite on the fire. There will be a fireball ranging from 1 to 3 feet in diameter. The explosion is harmless unless you are within a few feet then you may get burned. There is no force from the explosion, just a small wave of air and heat.