Take a potato and cut it into thin slices. Prepare a solution of salt water by adding two tablespoons of salt to half a litre of water, and stirring until the salt has dissolved.
Place half of the potato slices in a shallow dish of the salt water and place the rest in a shallow dish of plain tap water. Wait for at least 20 minutes. The longer you leave the potatoes the more noticeable the effect will be.
You should find that the slices in the salt water have gone limp. This is because of osmosis. The concentration of salt in the salt water was greater than that in the potato. Water molecules passed from the potato into the salt solution, decreasing the concentration of the salt solution, and leaving the potato limp.
To plump up the limp potatoes place them in a dish of plain tap water and leave them for 20 minutes. The water molecules pass from the dish into the potato, as the concentration of salt in the potato is greater than that in the water.
Another good way of exploring this effect is to take three half-potatoes.
Place each half, cut-edge down, in separate shallow dishes of water. Remove a small square of potato from the top of each of the half-potatoes. Put one teaspoon of salt into the recess of the first potato, one teaspoon of sugar in the second, and leave the third empty. Wait for a couple of hours, checking on the potatoes regularly.
You should find that water appears in the recess in the first two potatoes.
The same effect of osmosis is occurring, as water passes from the potato into the salt or sugar in the recess. This leaves a greater concentration of salt or sugar in the potato than in the water, and therefore water molecules pass into the potato from the water in the dish.
Sophie Duncan is programme manager for science at the BBC www.bbc.co.ukscience