Rain in a jar

Place a green twig in a glass of water in sunlight. Pour a layer of oil on to the surface of the water and invert a large jar over the lot. After a short time, drops of water collect on the walls of the jar. Since the oil is impermeable, the water must come from the leaves. In fact the water which the plant absorbs is given off into the air through tiny pores in the epidermis of the leaf. Air saturated with moisture and warmed by the sun deposits drops like fine rain on the cool glass.