Creating this flask (and learning about how liquids can be kept hot or cold) will help to develop your child's knowledge and understanding of the world around them, and how things work. It will also help to improve their hand-eye coordination and fine motor skills as they wrap the bottles in foil and then roll the inner bottle in bubble wrap and place it inside the larger bottle to create the flask. Your child may find some of this task tricky, so make sure you're on hand in case they get stuck.
How to extend the magic
Explain to your child that flasks are good at keeping drinks hot and cold for longer. Experiment with your homemade flask. Try adding some iced water into the inner bottle first and leave the bottle for half an hour. Go back and check - has the ice melted? Then put some warm water in the bottle and leave it for half an hour. What has happened - is it still warm?