Tempera paints are often runny because they are water-based. This is not a problem for your everyday craft projects. But if you want to create a painting with some depth you will need to thicken the tempera paint before using it. Fortunately, you can use a common household pantry item to thicken your tempera paint.
Mix 2 Tbsp of cornstarch in a small pot with 1 1/2 cups of water.
Heat the cornstarch and water mixture over low heat. Stir until the cornstarch is completely dissolved in the water. Remove it from the heat.
Wait for the cornstarch and water mixture to cool to room temperature. Add the mixture to the tempera paint of your choice a little bit at a time. Stop once you reach your desired thickness.
Store any extra cornstarch and water thickener in an airtight container.
You can also store the cornstarch thickener in a plastic zipper bag.