weight是名词，而疑问词中，what 是相当于名词的。

比如问你多重，可以问how weighty are you?(形容词）或what is your weight(名词）或how much do you weigh(动词）。再比如问你岁数，可以问how old(形容词）are you 或what is your age(名词）（这个问法不常用）。可以看出，what 用来代替名词，How是用来修饰形容词或副词的。