I bought our sponges at the Dollar Tree in a package of 3 for $1. I bought 4 packages and we made 6 balls (2 sponges per ball). A friend of mine said that she used cheap dollar store sponges and hers fell apart after a few uses. She recommended the O-cello brand sponges, which she bought at Target. We've used our "Sponge Bob" water bombs about 3 times and so far they are holding up great.
For bigger water bombs, cut the sponges into 3 pieces lengthwise. You will need 6 (or more!) total pieces for each ball. I also tried making some smaller balls using 6 pieces that had been cut widthwise (you'll get 4 per sponge...so only 1 1/2 sponges needed). I kinda prefer the smaller ones to play with. They don't look as "alien like" and my hubby says you get more control when throwing them. Jackson says the bigger ones are better because they hold more water. Again...make them however you like!
The secret to these guys is to use some heavy duty dental floss, fishing line or ribbon and to tie them as tight as you can get them. My friend used zip ties on hers, but I didn't have any of those (and I think they might hurt if you got hit with one...but I am a weenie that way!).
Stack the pieces up, putting some flat and some on their sides, and tie them super tight. Wrap both of the ends around the other way and tie again into a knot. You want them to be very secure so the sponges don't fall apart after the first game. Cut the excess ribbon or string off, leaving a little bit of a tail so it doesn't unravel (especially if using ribbon).
That's it!! It takes about 5 minutes to make these things unless your child has more fun playing with the uncut sponges than the water bombs. LOL!
The only thing left to do, is head outside with a couple buckets of water and have a blast! After the second day of playing with the "Sponge Bob" water bombs, Jackson came up with a few new "moves". He likes to squeeze them out over my husband's head and he also did a fun "squeeze and spin" move that created a crazy spray of water (and you didn't have to chase the ball!).
Let them dry out when you're done and you'll have hours and hours of inexpensive fun (without filling a single obnoxious water balloon!).
Happy first day of summer!