Fill the balloon with about 3/4 cup of water. You want it to be the size of a baseball so you might need to add a little air. Tie it and then tie off the rubber band on the knot.
My kids loved playing with these. It occupied them for an hour! Once the rubber bands broke, they wrapped up their balloons in a blanket and played baby.
And then they smashed the balloons on the ground (but only after they were thru playing house with them!)
I’ve heard that these can be dangerous with the rubber bands popping kids or being tied around things. Please make sure you supervise your kids and then dispose of these homemade toys when they are done.