Take a long length of a cardboard, roll and stick the ends together with gum. Now with small paper pieces of the same height as the card roll, you make small rolls of minute radius.
Insert all of them into the big roll, until they are all tight. Now look at the size of the big roll.
You will think that the space inside the big roll is very small. After making this observation, pull out all the small rolls and spread them on your table. You will now be surprised by the vastness of the total surface area of the little papers that were inside the big roll. In this way you can compare the vastness of the surface areas of the oxygen absorbing tissues inside the lungs.
Digestion of Fats
Fats don't mix or dissolve in water. But the bile fluids secreted by the liver help in the digestion of fats. To demonstrate a similar effect, take two glasses of water and add a spoon of ghee to both. You will find that ghee doesn't mix with water.
Now to one of glass add a little soap and stir well. You will find that the ghee and the water mix and turn into a white coloured emulsion.