One day, a teacher was giving a speech to his students .He held up（举起）a glass of water and asked the class. How heavy do you think this glass of water is ?The students' answers ranged（范围） from 20g to 500g .It does not matter on the weight itself .It depends on how long you hold it .If I hold it for a minute ,it is OK. If I hold it for an hour ,I will have an ache in my right arm .If I hold it for a day ,you will have to call an ambulance（救护车）.It is exactly the same weight ,but the longer I hold it ,the heavier it becomes.If we carry our burdens （负担）all the tim