This one is very simple, it may take some time to make.
Materials:
Paper caps
Scissors
Scotch tape
Step 1: Take some paper caps and cut each little cap section off and make a pile of 20 or so.
Step 2: Now stack the caps together one on top of the other.
Step 3: Once there aligned and stacked, take a small piece of tape and wrap the tape around the caps.
Step 4: Take the caps and stick them somewhere and take a bb gun and pump it 15 times. And shoot the caps.
You will be rewarded with nice blast. The noise is quite loud for the small size. you may ask why can't I just roll the caps in a ball and shoot it, well that's because when you stack them on each other the explosion is better because you hit all the caps. Opposed to some of them with the tape and roll method.