This is a really simple manual on how to make my favorite DIY toy as a child.
Basically it is just some sort of a slingshot, however, made by everyday common items (and doesn't require carving in wood of any kind).

Supplies

1. Plastic bottle.

2. Balloon.

3. Bullets - any small material up to 1 cm in diameter (small rocks, peas, beans, etc).

4. Scissors.

Step 1: Cut

Take the scissors and cut the bottle top (bottleneck part) off the bottle.
You should be left with a piece of plastic shaped like a cone (see attached picture).

Step 2: Apply Balloon

Apply the balloon onto the bottle cap opening, using one hand to expend the balloon lips, and the other hand to push the bottle top in.
Push it is far as you can go.

Step 3: Shoot

1. Put a bullet (small pea) into the balloon.
2. Use one hand to hold tightly on to the bottle cap opening.
3. Use the other hand to pull the balloon backwards (recharge it).
4. Aim at a sitting, non living, target.
5. Release the grip on the balloon and see the bullet flies (and hope for a hit).

If you didn't hit, repeat instructions 1-5 until you do.
Enjoy!