One of the simplest yet most amazing ideas ever conceived. The plastic bag cannon. The idea originated at my lunch table at high school, where I would discretely fire off rounds of gummies and head-shot unaware freshman.
-One zip-lock 3oz plastic bag
-Projectiles, preferably round and eraser sized.
-M n' M's
-Small bits of fruit
Step 1: Preparing the Cannon
Inflate the plastic bag about 2/3rds of the way. If the bag is not full enough, or too full, the launch will not be powerful.
Slide the projectile about two inches into the corner of the bag.
Step 2: Firing the Bag
Aim the bag at the intended target, and slam down! Not too hard though, or the bag will pop. Experiment with different forces and see what makes the fastest projectile. Have fun, and don't hurt anyone. :D
The bag will probably last for about 30 shots, possibly more. Below are videos of me testing it.