Secret path

Dissolve a teaspoonful of salt in a glass of water and cover it tightly with parchment paper. Place the glass upside down in a dish containing water strongly coloured with vegetable dye. Although the parchment paper has no visible holes; the water in the glass is soon evenly coloured. The tiny particles of water and dye pass through the invisible pores in the parchment paper. We call such an exchange of liquids through a permeable membrane, osmosis.