Mix the baking soda, citric acid, corn starch and salt together in a medium-sized bowl.
You can use a spoon or pestle to break up any chunks. Then whisk together.
Mix the oil and water together and very slowly add it to the dry ingredients. Mix with a whisk as you add the wet ingredients. Add the wet ingredients slowly so you do not cause too much fizz when they are added.
Your mixture will hold together like wet sand once it is fully mixed. Add a small amount of water if the mixture still seems too dry.
If you want to make your bath bombs multiple colors, divide the mixture up into separate bowls. Add a few drops of food coloring to each bowl and mix well. Use the food coloring sparingly to avoid issues with staining.
Overfill one side of the bath bomb mold with the mixture.
For the other side of the bath bomb mold, add some mixture and then press a Shopkin or another small toy into the mold and fill with more of the mixture. You want both sides of the mold to be heaping full.
Firmly press the 2 sides of the mold together.
Allow the bath bomb to sit for a few minutes and then give the mold a tap with a spoon and carefully remove it to reveal the bath bomb. If the bath bomb breaks apart, simply add back into the mold and try again.
Let the bath bomb dry overnight.
When you are ready to use, simply drop the bath bomb into warm water and watch it fizz and reveal a surprise toy! Make sure to take the toy out of the water before draining the tub.