Use a mouthwash or water bottle; a mouthwash bottle is best as it has the lid which is child-safe, and too challenging for little hands to twist open. If you’re using a bottle that’s not child-safe, you might want to run a little hot-glue around the inside of the bottle lid.
Now, simply top up the bottle with oil. I filled ours right to the top and then popped the lid on.
Tip the bottle upside down and watch your ocean come to life.