Take four peeled potato halves and scoop each one out to make potato cups. One of these potato cups should be made from a boiled potato. Put each potato cup in a trough containing water.
(a) Keep cup A as empty.
(b) Put one teaspoon sugar in cup B.
(c) Put one teaspoon salt in cup C.
(d) Put one teaspoon sugar in the boiled potato cup D.
Keep these for two hours. Then observe the four potato cups and answer the following.
Explain why water gathers in the hollowed portion of B and C.