Get them to step on it is one way
You'll need:
1 balloon
1 knife or scissors
1 plastic bottle
(optional) some tape
There is a label on the bottle cut above that you want the part where the lid is.
Take off the lid toss it take the freshness seal thing off too with scissors or a knife.
Put the balloon on the neck of the bottle where the lid was.
Make sure the neck of the balloon is on the neck of the bottle
Now if you want tape the balloon on to the lid.
Put the marble into the balloon pull back and shoot. Ta Da a lethal weapon made with normal material.
*Note if you shoot someone with this you'll probably kill them just so you know
