Take a small enamelled bowl and fill it with water. When all the ripples have died away and there is no movement, gently place a matchstick on the surface, in the centre. An inch from the middle of the match, suspend a lump of sugar so that it is partly submerged. This can be done by running a piece of cotton around the sugar. Do all this as carefully as you can, because you must not agitate the water.
In a few moments the match will be attracted to the lump. Why? There is, of course, no real attraction such as results between a magnet and steel pen-nib. The reason is a curious one. The sugar dissolves and makes a sugary solution which is heavier than pure water. Accordingly, it sinks and is doing so leaves an empty space. But no sooner is the space framed than the surrounding liquid rushes in to fill the void. In rushing to the space the water carries with it the match stick.