Get your two balloons. I am using water balloons but this would also work with normal balloons.
Put your inside balloon on the straw.
Slide the other balloon over the top.
Slide the balloons off the straw keeping one inside the other.
Place them on your filling device! I am using a water pump but you could easily use a tap.
Fill your balloon up, the inside one should inflate! Don't make it too big or you won't be able to fill the outside one.
Tie the inside balloon, the one you just inflated!
Push the knot inside and place the neck of the outside balloon back on your filling device.
Inflate the balloon then knot it. Your double water bomb is now complete!
When you throw the bomb the outside balloon pops but the inside one doesn't! :-D
This is one of my very first guides so please comment what you thought and if you have any questions!!!