1.Peter performs a magic to Mary.He puts a piece of paper on the top of a glass which is full of water and presses it against the opening of the glass.Then,he turns the glass upside down.When he withdraws his hands from the paper,the paper and the water do not fal.Can you explain why the water and the paper do not fall?
2.Can Peter use a glass full of sand instead of water to perform this magic?Why?
(第二題唔識答唔緊要)