Add a few drops of liquid watercolour paint or food colouring and mix them in with a spoon.
Stir in baking soda.
Add 1 tablespoon of contact lens solution and stir with a spoon. Slowly add the rest of the contact lens solution, mixing well and making sure to stop before the slime gets too rubbery.
Remove the slime from the bowl and knead it with your hands. It will start out sticky, but the more you knead it, the less sticky it will feel.
When the slime no longer feels sticky, add the aquarium rocks and gently mix them in with your hands. Once you're satisfied with the consistency of the slime, give it to your child to play with!
Notes
Make sure the contact lens solution you use lists “boric acid” and “sodium borate” as ingredients. If those ingredients are not listed, the slime will not form and the recipe will not work.