Combine bicarb soda and cornflour in a bowl. Mix well.
In a separate cup/container, combine oil and food dye. Stir with a fork to break up the blobs of food dye (they won't mix well with the oil, that's fine).
Pour the oil into the dry ingredients and mix to combine. You should end up with a soft coloured sand that will hold its shape when pressed firmly together.
Pour into a tray for your kids to play with.
When you want to make some fizz, get a syringe or spoon for your little ones to add vinegar to the sand and watch it fizz up and bubble!
Once it has stopped fizzing, the slime that is left is fun to mix up and watch as it thickens then relaxes, depending on how much it is moving.