1. Mix a cup of water with two cups of cornstarch. Stir until goopy. You should feel resistance when stirring quickly, and no resistance when stirring slowly. This is because the oobleck mixture is a non-newtonian fluid and turns solid when pressure or force is applied.
2. Pour the oobleck mixure into the water bottle.
3. Attach the opening of the balloon to the top of the water bottle. Flip over and squeeze the contents into the balloon until full.
4. Taking care not to let any air bubbles get into the balloon, take the balloon off and tie it in a knot.
5. Take another balloon and cut the end off. Wrap this over the filled balloon to cover up the knot and add more durability.