1. In a bowl, mix the citric acid, cornstarch, Epsom salt, baking soda, water, coconut oil and lavender essential oil.

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2. Separate the mixture into three small jars and add food colouring to get the colours you want.

3. In a fourth bowl, mix a small amount of your three different colours to create a galaxy effect. Fill both parts of your mould, pressing down well. You want to make your mixture as compact as possible in the mould. Then, to make the two halves stick together, apply a thin layer of coconut oil before closing your mould.

4. Let the bath bomb air dry for 2 hours before removing it from the mould. You can also place it in the refrigerator to solidify.

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PSTT!

If you don't have a spherical mould, you can also fill an ice-cream scoop with the mixture and let it dry in the scoop before unmoulding. You'll get a nice half circle!