Add 2 tablespoons of water and work it into the cornstarch with your hands until it has spread throughout. Repeat with a third tablespoon of water. If needed, repeat with a fourth tablespoon of water. The mixture should have a dough-like consistency.
Add ½ teaspoon of vegetable oil and knead it into the dough using your hands. Repeat with a second ½ teaspoon of vegetable oil. (You can skip the second ½ teaspoon if the dough is too sticky.)
Test the play dough's consistency. It should be a bit crumbly, but easy to shape into a ball. If the dough is too sticky, add a bit more cornstarch. If the dough is too dry, add a bit more water by wetting your hands and kneading until the water from your hands has made its way into the dough. Play immediately.
Keyword cornstarch, jell-o, play dough, sensory play, taste-safe