MySQL comparison operator in problem solving

About comparison operators IN
I read a Book 、 Tutorial video 、 online , It's all introduced like this ：
Judge whether the given value is IN A value in the list . If it is , return 1, Otherwise return to 0. If the given value is NULL, perhaps IN The list exists NULL, The result is NULL.

But SELECT 1 IN (1, NULL) The result is 1, instead of NULL.

Why is that ？ Not to say that perhaps IN The list exists NULL, The result is NULL Well ？？