Add 10-20 drops of food colouring and mix some more, until the colour is visible throughout.
Cover the bowl or container and place it in the fridge. Let the chia seeds soak for 3-4 hours or overnight, until they develop a thick, slimy texture.
Add 1 cup of cornstarch and stir with a spoon until fully incorporated. Repeat with a second cup of cornstarch. Add an additional ½ cup of cornstarch and knead the slime with your hands until fully combined, then repeat with another ½ cup of cornstarch.
If you are satisfied with the texture of the slime, give it to your child to play with. If not, add a couple more tablespoons of cornstarch at a time, kneading with your hands, until you are happy with the texture. Use immediately.
Notes
Slime can be stored in a container in the fridge for up to 5 days when not in use. In order to reuse the slime, you will need to add more water and work it in with your hands.