Homeostasis - Osmoregulation
The experiment from this website has been tried and works well: http://www.nuffieldfoundation.org/practical-biology/investigating-effect...
The experiment involves identifying how much Ribena needs to be diluted before it becomes isotonic to potato cubes.
Students then do their own investigation with a fluid of their own choice (i.e., lemonade, orange juice, etc.) to determine the correct dilution to give to a dehydrated child (i.e., with the same solute concentration as the potatoes).