Mix the cornstarch and oil together in your measuring cup, slowly add the water until it becomes a liquid. Once it is smooth and runny, pour into an empty water bottle.

Secure a balloon around the mouth of the water bottle and invert to pour the mixture into the balloon. Be careful to not let any air get in the balloon, as this will likely cause it to pop when squeezing it later. Squeeze the bottle to get all the mixture into the balloon. Carefully remove the balloon off the bottle and tie a knot to seal it.

Depending on the quality of your balloon, you may want to double wrap your stress ball. Take a second balloon and cut the rolled back end off so you can more easily stretch it over your ball, tie a knot to secure it.

Now you are ready to play! Enjoy!